Complete the reporting refactor to use internal.Reporter.

The partest blocker was fixed in v1.0.14. sbt and IDE
are not validated, so reporters.Reporter is not removed.

There are internal dependencies on cancel and comment
functionality, which is resolved here by typecasing.
Both features need to be implemented otherwise.

ConsoleReporter is still a reporter.Reporter.

(cherry picked from commit d59389e)

(cherry picked from commit e7389e0)

The extended explanation attached to the first feature
warning would fool the reporter if the feature is reported
again at the same position.

Use a conventional leading `"----"` to demarcate the
explanation, and use only the prefix to see if the error
has already been issued at a position.

(cherry picked from commit 6a27388)

(cherry picked from commit 7ca0a7f)

This can be handy for retaining source compatibility. Or for when you
just know better.

Fixes scala/bug#8908

(cherry picked from commit ee18631)

(cherry picked from commit 9ced2d8)

ContextReporter no longer extends Reporter. The ContextReporter is
only used in the context, and it reports through Global.reporter. But
it's never assigned to Global.reporter. So better keep the two
separate. This also saves the inherited fields (ContextReporters are
allocated manies, type checking every Apply uses silent).

Remove most reporter mixins to get to a linear inheritance structure,
no more delegating reporters.

Ignore the `force` parameter of `info0`.

Some quirks to remain source compatible with the sbt compiler interface.
`nsc.reporters.Reporter` only exists for sbt compatibility, Global
has a `FilteringReporter`. `-Xreporter` must be a FilteringReporter.

(cherry picked from commit 5f6b28c)

(cherry picked from commit b13d671)

(cherry picked from commit 3e7e0ab)

(cherry picked from commit 2cf8e5c)

(cherry picked from commit 60fcf45)

(cherry picked from commit 1489e59)

(cherry picked from commit 670a55c)

(cherry picked from commit 251e4a5)

- echo without Position parameter to ContextReporter, used in splain
- StoreReporter.Info type/value aliases (scalameta, scapegoat)
- make `info0` deprecatedOverriding instead of final (scalameta)

(cherry picked from commit b5d951b)

Even though these warnings are emitted in the typer phase, the actual
invocation is done at the end of type checking in the root context.
So we should not use the `context.warning` method, as it injects
`context.owner` as the warning site, which is always the root symbol.

(cherry picked from commit a466056)

Previously, hasWarning meant a warning had been displayed
(as opposed to counted). Since last refactor, it means
counted, so warnings were summarized under -nowarn.
Until 2.13.1, there was no summary under -nowarn.

Let -nowarn mean -Xmaxwarns 0 (to mean show me no warnings)
and additionally don't summarize any warnings (including
erroring under -Werror).

Named constants for Reporter.filter

Test that suppression is relaxed in debug mode.

Restore maxwarns after suppressing

Scaladoc also touches nowarn

ConsoleReporter respects nowarn, since PerRunReporting
suppression mechanism has a checkpoint after typer,
such that subsequent warnings are issued directly and
therefore bump the warnings count.

(cherry picked from commit 2a2f416)

If there is an error running a forked command, such as
a Test or javac, log it instead of letting sbt swallow
the exception. (It offers to let you run the last command
to see the stack trace.)

This helps if javac goes missing.

(cherry picked from commit f342293)

Previously, a bad compiler option was ignored.

Add a common idiom to StreamCapture to set stdout;
the idiom may be suboptimal for testing.

Convert a partest to junit for regex, and eliminate
one check file.

(cherry picked from commit 580ed07)

Remove withDefault.

OutputSetting is StringSetting loosely coupled
to outputDirs.

MultiStringSetting takes a default

Back out clearSetByUser, which worked around initialization
of the setting.

(cherry picked from commit 649fea5)

(cherry picked from commit 0c52dc4)

Backport from 01ea4f8