fix(bzlmod): correctly wire the extra_pip_args

[aignas]

Before this PR we were just dropping the extra_pip_args passed to
pip.parse and were just using the args passed through the requirements
file. Thanks to @swarren12 for pointing this out.

This PR also passes extra_pip_args to sdist whl_library instances
so that users can build the sdists correctly when using
experimental_index_url feature.

Summary:

• pass extra_pip_args when building sdists in experimental mode
• join extra_pip_args from the file and the pip.parse attr
• test: add a test to ensure that the extra args are joined

Fixes #2239
Closes #2254