refactor(pkg_aliases): create a macro for creating whl aliases

[aignas]

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

[aignas]

Bazel 9 integration tests are failing - I'll wait for the #2379.