…-contrib#2391)

This just cleans up the code and moves more logic from the
repository_rule
(i.e. generation of `BUILD.bazel` files) to loading time (macro
evaluation).
This makes the unit testing easier and I plan to also move the code that
is
generating config setting names from filenames to this new macro, but
wanted to
submit this PR to reduce the review chunks.

Summary:
- Add a new `pkg_aliases` macro.
- Move logic and tests for creating WORKSPACE aliases.
- Move logic and tests bzlmod aliases.
- Move logic and tests bzlmod aliases with groups.
- Add a test for extra alias creation.
- Use `whl_alias` in `pypi` extension integration tests.
- Improve the serialization of `whl_alias` for passing to the pypi hub
repo.

Related to bazel-contrib#260, bazel-contrib#2386, bazel-contrib#2337, bazel-contrib#2319 - hopefully cleaning the code up
will make
it easier to address those feature requests later.

---------

Co-authored-by: Richard Levasseur <richardlev@gmail.com>

…requirements files (bazel-contrib#2377)

This change makes it possible to handle local versions of packages,
which is extremely useful with PyTorch. With this change, it is
possible to have different local versions of the same package in the
`requirements.txt` file translated to valid `whl_library` repositories.

Fixes bazel-contrib#2337