feat(pipstar): parse entry_points without Python

[aignas]

This is a small utility function to get us Python free when wheels are
extracted in the repository phase.

Next is to extract the wheel using repository_ctx.extract (#3430).

Whereas patching the wheel after extracting is more involved to be done
without Python because we need to rezip the wheel and that has to be done
with Python for a few reasons (to stay sane). If we want to remove this,
then we would have to create a whl file in the build phase, which could
work, but will need to be an exercise for the reader.

Nevertheless, this moves us towards removing any side-effects from Python
interpreter, so changing the default interpreter would not cause us to
refetch everything.

[gemini-code-assist]

Summary of Changes

Hello @aignas, I'm Gemini Code Assist¹! 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 significantly advances the pipstar initiative by enabling the parsing of Python wheel entry_points without relying on a Python interpreter. It achieves this by introducing a new, robust mechanism to directly read and interpret the entry_points.txt file within a wheel's metadata. The changes involve refactoring the core whl_metadata function to incorporate this new parsing, adjusting the wheel extraction flow for pipstar mode, and adding comprehensive tests to ensure the accuracy of the new parsing logic.

Highlights

• Python-Free Entry Point Parsing: Introduced a new utility function to parse entry_points.txt directly from Python wheel metadata, eliminating the need for a Python interpreter during this specific step. This is a key part of enabling a more 'Python-free' wheel extraction process for the pipstar initiative.
• Refactored Wheel Metadata Handling: The whl_metadata function has been enhanced to proactively read entry_points.txt (if present) and integrate its parsed content into the overall metadata struct. This streamlines how entry points are accessed downstream.
• Conditional Wheel Extraction Logic: Modified the _extract_wheel function to include an early return when enable_pipstar is true, allowing pipstar to bypass certain dependency and metadata processing steps. This suggests a distinct, optimized path for pipstar's wheel handling.
• Enhanced Test Coverage: Added a dedicated test case to thoroughly validate the new entry_points.txt parsing logic, ensuring correct extraction of names, modules, attributes, and extras, even with comments present.

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[gemini-code-assist]

Code Review

This pull request introduces a Starlark-based parser for entry_points.txt to avoid shelling out to Python, which is a great improvement for performance and hermeticity. The changes are primarily in whl_metadata.bzl with corresponding updates in whl_library.bzl and tests.

I've found a critical issue where a change in wheel_installer.py will cause failures in whl_library.bzl due to a leftover call. I've also identified a bug in the new entry points parser logic and a minor inaccuracy in a docstring. My review includes suggestions to fix these issues.