feat: add --debugger flag

[rickeylev]

The --debugger flag is useful for injecting a user-specified dependency without
having to modify the binary or test. Similarly, tests now implicitly inherit the
PYTHONBREAKPOINT environment variable.

The dependency is only added for the target config because build tools can't be
intercepted for debugging.

[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 enhances the debugging experience for Python binaries and tests by introducing a new --debugger flag, which allows for flexible injection of custom debugger dependencies. It also improves the automatic inheritance of the PYTHONBREAKPOINT environment variable, making breakpoint() usage more seamless. Alongside these user-facing features, the PR includes substantial internal refactoring to streamline the underlying rule implementations, making them more robust and easier to manage.

Highlights

• New --debugger flag: Introduced a --debugger flag for py_binary and py_test rules, allowing users to inject a custom debugger dependency without modifying the target's deps attribute. This dependency is only added for the target configuration, not for build tools.
• PYTHONBREAKPOINT inheritance: Tests now implicitly inherit the PYTHONBREAKPOINT environment variable, streamlining the use of custom debuggers with breakpoint() functionality.
• Internal Refactoring: Significant internal refactoring was performed in common.bzl, py_executable.bzl, and py_library.bzl to simplify semantics structs and remove redundant parameters, improving code maintainability.
• Configuration Management: Added new configuration settings and a bazel_config_mode rule to differentiate between exec and target configurations, ensuring the debugger dependency is applied correctly.

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[gemini-code-assist]

Code Review

This pull request introduces a --debugger flag to inject a debugger dependency into py_binary and py_test targets. It also makes py_binary and py_test targets implicitly inherit the PYTHONBREAKPOINT environment variable. The implementation is clean and includes documentation and tests for the new flag. A significant part of this PR is a refactoring that removes the semantics struct pattern from py_executable.bzl and py_library.bzl, simplifying the code by removing a layer of indirection. The changes look solid. I have one minor suggestion for improving the accuracy of the changelog.

[aignas]

I see that the debugpy can be used either as a CLI via python -m debugpy which could work well with our main_module or via import debugpy that would automatically work with this as well. To be truly useful, it would be nice to give at least one example how to setup the debugger. That can be done in a separate PR though.

[rickeylev]

What does python -m debugpy do? Just start a debugpy server?

debugpy is particularly interesting because its a client/server design, so it can can actually Just Work with bazel test (where stdin is gone).

[rickeylev]

re: example for how to use them: I added a basic howto doc.

[aignas]

Thanks!