feat(pypi): actually start using env_marker_setting (#2873)

Summary:
- `pep508_deps` is now much simpler, because the hard work is done in
  analysis phase
- `whl_library` BUILD.bazel tests now also have a test for the legacy
  flow.

One thing that I noticed is that now we have an implicit dependency on
the python toolchain when getting all of the `whl` target tree. This is
a filegroup target that includes dependent wheels. However, we fallback
to the flag values if we don't have the toolchain, so we should be good
in general.

Overall I like how this is turning out because we don't need to pipe the
`target_platforms` anymore when we enable `PIPSTAR` feature. This means
that we can start creating fewer whl_library instances - e.g. a
`py3-none-any` wheel can be fetched once instead of once per python
interpreter version. I'll leave this optimization for a later time.

Work towards #260

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