test(bzlmod): refactor pip_parse to make it more testable

[aignas]

This PR introduces a new parse_modules function in the pypi/extension.bzl code to mimic the structure of the python extension and to make it easier to write unit tests against the extension itself. I have also written a few unit tests to verify the generic structure.

With this we can:

• Write unit tests to see what whl_library repos would be created.

This is just making the seam and the tests, etc will be written in followup PRs.

Work towards #2268.