chore: add some type information about hub builder

[rickeylev]

While reading hub_builder.bzl, I had to track down a lot of source to figure out
what variables were tracking what.

Add various comments to describe variable types and their purpose.

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, 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 improves the readability and maintainability of the hub_builder.bzl and extension.bzl files by introducing extensive type information and documentation comments. The changes aim to make it easier for developers to understand the purpose and structure of various variables and function arguments, reducing the need for deep source code tracing and enhancing overall code comprehension.

Highlights

• Improved Type Information for pip_hub_map: Added a type hint comment for the pip_hub_map variable in extension.bzl, clarifying its structure as dict[str repo, HubBuilder].
• Enhanced hub_builder Variable Documentation: Introduced comprehensive type hint comments for numerous internal variables within the hub_builder function in hub_builder.bzl, significantly improving the understanding of their data structures and purpose.
• Clarified _build Function Return Structure: Enhanced the _build function's return struct with explicit type comments for its fields, such as whl_map, group_map, extra_aliases, exposed_packages, and whl_libraries, providing clearer expectations for its output.
• Added Docstrings to Setter Functions: Incorporated detailed docstrings, including argument types, for setter functions like _add_exposed_packages, _add_group_map, _add_extra_aliases, and _add_whl_library, thereby improving the API documentation and usability of these functions.

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[gemini-code-assist]

Code Review

This pull request is a great initiative to improve code readability by adding type information in comments. I've found a few instances where the added type hints appear to be incorrect. My review includes suggestions to correct these, ensuring the documentation is accurate and helpful. The rest of the changes look good.

[aignas]

Thank you!