feat: support per-wheel enable_implicit_namespace_pkgs in whl_mods (WIP)

[chrisirhc]

Update: I was reading a thread in #python and saw that this set to True when setting --incompatible_default_to_explicit_init_py is actually desirable and forward-looking.

So perhaps this PR is unnecessary for my use case, unless somebody runs into issues and needs to toggle it for some packages/wheels.

Other note: perhaps the enable_implicit_namespace_pkgs documentation can be further clarified.

---

As part of investigating https://github.com/aspect-build/rules_py/issues/611 which requires enable_implicit_namespace_pkgs = True as a workaround to unblock PEX creation, I looked into how to only toggle on this flag per wheel, as per https://github.com/aspect-build/rules_py/issues/611#issuecomment-3305979729 .

The goal of this is to reduce reliance on this flag where it's not necessary.

This PR is a proof-of-concept to gather if there's any interest in this feature. The commits aren't cleaned up yet.

Not yet done:

• [ ] Update CHANGELOG.md
• [ ] Update documentation for whl_mods

[gemini-code-assist[bot]]

Summary of Changes

Hello @chrisirhc, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant feature to rules_py by enabling the enable_implicit_namespace_pkgs flag to be set individually for each wheel via whl_mods. This change aims to provide more precise control over how namespace packages are handled, specifically addressing issues related to PEX creation by allowing the flag to be enabled only where necessary, thereby reducing its global reliance. This is currently a proof-of-concept to gather feedback on the approach.

Highlights

• Per-wheel Namespace Package Control: Introduced a new enable_implicit_namespace_pkgs attribute to whl_mods and package_annotation, allowing granular control over __init__.py file generation for namespace packages on a per-wheel basis.
• Configuration Propagation: The new enable_implicit_namespace_pkgs setting is now propagated through the pypi extension and utilized when generating whl_library build rules, ensuring the per-wheel configuration is applied.
• Test Coverage: New test cases have been added in extension_tests.bzl to verify the correct handling and propagation of the enable_implicit_namespace_pkgs attribute within whl_mods, and existing tests in generate_whl_library_build_bazel_tests.bzl were updated to include this new parameter.

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