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

---------

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

This makes the python bzlmod extension handle generating the
platform-specific toolchain
entries ("python_3_10_{platform}"). This is to eventually allow adding
additional
toolchains that aren't part of the PLATFORMS mapping in versions.bzl and
have
their own custom constraints.

The main things this refactor does are:
1. The bzlmod phase passes the full list of implementation toolchains
to create (previously, it relied on `hub_repo` to generate the
implementation
   names).
2. The name of a toolchain (the toolchain.name arg) and the repo that
implements
it (the py_toolchain_suite.user_repository_repo arg) are separate. This
allows
   future work to mixin toolchains that point to arbitrary repos.
3. The platform meta data uses a list of target settings instead of dict
of
flag values. This allows more arbitrary target settings. For now, flag
values
on the platform metadata is still looked for because it's known that
users
   patch python/versions.bzl.

Along the way:
* Factor out a platform_info helper in versions.bzl
* Factor out a NOT_ACTUALLY_PUBLIC constants to better denote things
that
  are public visibility, but actually internal.
* Add some docs to some internals so we don't have to chase down their
definitions.

Work towards #2081