This PR ensures that environment activation sets all environment variables set by the equivalent `module load` operations, except that the spec prefixes are "rebased" to the view associated with the environment.

Currently, Spack blindly adds paths relative to the environment view root to the user environment on activation. Issue spack#12731 points out ways in which this behavior is insufficient.

This PR changes that behavior to use the `setup_run_environment` logic for each package to augment the prefix inspections (as in Spack's modulefile generation logic) to ensure that all necessary variables are set to make use of the packages in the environment.

See spack#12731 for details on the previous problems in behavior.

This PR also updates the `ViewDescriptor` object in `spack.environment` to have a `__contains__` method. This allows for checks like `if spec in self.default_view`. The `__contains__` operator for `ViewDescriptor` objects checks whether the spec satisfies the filters of the View descriptor, not whether the spec is already linked into the underlying `FilesystemView` object.

Fixed #15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see #13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).

Fixed #15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see #13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).

Fixed #15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see #13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).

Fixed #15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see #13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).

Fixed #15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see #13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).

Fixed spack#15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see spack#13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).