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