refactor: read migrated native flags through a centralized accessor function (#3290)

This lets Python logic support either Starlark- or native-defined flags,
based on Bazel support:

- Pre-Bazel 9.0: always use native flags
- Bazel 9.0+: use Starlark flags if [incompatible
change](https://github.com/bazelbuild/bazel/pull/27056) that disables
`ctx.fragments.py` and `ctx.fragments.bazel_py` is set
- Include a developer override to trigger Starlark versions while
testing Starlarkification. Developers enable this by updating .bzl code
in their local workspace. This can be removed as soon as
Starlarkification is complete

Also check in a Starlark version of
`--experimental_python_import_all_repositories` for testing.

**This is a no-op. New flag sources of truth don't take effect without a
supporting bazel version that sets appropriate incompatible flags.**

**Caveats:**

- May not work with `configuration_field` as implemented. We can loosen
the incompatible flag lockdown if that's an issue.

For https://github.com/bazel-contrib/rules_python/issues/3252

TODO list

* [ ] Add docs to docs/api/rules_python/python/config_settings/index.md
for starlarkified flags