refactor: remove most of semantics

[rickeylev]

This removes the majority of the "semantics" plugin objects. These are a holdover
from making the code more amenable towards being extended by the Google
implementation.

py_library semantics are completely removed. Most of py_binary has had semantics
removed. I've left the debugger, build data, and cc deps hooks in for now, since
that functionality can still be folded into the code.

[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 undertakes a significant refactoring effort to remove the majority of "semantics" plugin objects from the Python rules. These objects were initially introduced to facilitate extensions for an internal Google implementation but are no longer serving their original purpose effectively. The py_library rule has been completely decoupled from this semantics system, and the py_binary rule has seen its semantics substantially streamlined, retaining only specific hooks for debugging, build data, and C++ dependencies. The overall goal is to simplify the codebase by removing an obsolete abstraction layer.

Highlights

• Complete Removal of py_library Semantics: The LibrarySemantics struct and all associated logic have been entirely eliminated, simplifying the implementation of the py_library rule by removing an unnecessary abstraction layer.
• Streamlined py_binary Semantics: The BinarySemantics struct has been significantly reduced in scope, with many parameters and functions removed. It now retains only essential hooks for debugger support, build data generation, and C++ dependencies.
• Direct Function Calls: Functions previously accessed through the semantics object, such as get_imports, maybe_precompile, and is_stamping_enabled, are now called directly. This change reduces indirection and simplifies the codebase.

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[gemini-code-assist]

Code Review

This pull request is a significant refactoring that removes the majority of the "semantics" plugin objects, which were a holdover for Google-internal extensions. The changes are well-executed and align with the goal of simplifying the codebase for open-source contributors. py_library semantics are completely removed, and py_binary semantics are greatly reduced, with only a few hooks remaining for debugger, build data, and C++ dependencies, as described. This makes the code more direct and easier to follow by removing a layer of indirection. The changes are consistent across all modified files and I couldn't find any issues. This is a great improvement for maintainability.