refactor(pypi): print a better error message for duplicate repos

[aignas]

Before this PR the error message would not be super helpful and may
potentially make it hard to debug and report errors. This PR does the
following:

• Add a better error message which also adds comparison of the args with
  which we create the whl library.
• Add a test that ensures that the error message is legible and works.
• Add the necessary plumbing to logger to allow for testing error
  messages.

A proper fix requires more work, so just adding better logging and error
messages may be useful here.

Work towards #3479

[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 refactors the PyPI module's error handling for duplicate whl_library definitions. The primary goal is to provide more actionable and detailed feedback to users when such conflicts occur, significantly improving the debuggability of build configurations. It achieves this by introducing a dictionary comparison utility and integrating it with an enhanced logging mechanism, ensuring that conflicting arguments are clearly presented in the error output.

Highlights

• Improved Error Messaging: The error message for duplicate PyPI whl_library entries has been enhanced to provide detailed comparisons of the arguments that led to the conflict, making debugging easier.
• Enhanced Logger Functionality: The internal logger utility (repo_utils.bzl) now supports custom printer functions and includes a failed() method to check if any fail messages were logged, which is crucial for testing error conditions.
• New Test Case: A new test (_test_err_duplicate_repos) has been added to specifically verify the clarity and content of the improved error message when duplicate whl_library declarations with different arguments are encountered.

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[gemini-code-assist]

Code Review

This pull request significantly improves the error message for duplicate repository definitions, which will be very helpful for debugging. The changes to the logger to facilitate testing are well-designed. I've left one comment with suggestions to refine the new _diff_dict utility function by improving its documentation and removing an unused variable. Overall, this is a great enhancement.