feat(migrate/python): use BUILD.bazel for Python libraries

[jskeet]

Uses BUILD.bazel files on all APIs associated with a library to:

• Pick up Python-specific GAPIC generator arguments
• Pick up when rest_numeric_enums is false (specified in the same way)
• Detect when the transport varies by API or is consistently not grpc+rest
• Detect non-GAPIC API paths

Some of this may become obsolete due to #3897 but we can address that when everything has landed.

Fixes #3407.

[jskeet]

(This will be a draft until #3890 is merged, at which point I'll rebase and there'll be a single commit when I request a review.)

[gemini-code-assist]

Code Review

This pull request introduces functionality to use BUILD.bazel files for configuring Python library generation during migration. The changes are well-implemented and include comprehensive tests. My feedback focuses on improving error handling by wrapping errors with more context to aid in debugging. Overall, the implementation is solid and aligns with the project's coding style.

[codecov]

Codecov Report

❌ Patch coverage is 93.44262% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.47%. Comparing base (5d0221a) to head (750ecc0).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
tool/cmd/migrate/python.go	93.44%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3893      +/-   ##
==========================================
+ Coverage   83.37%   83.47%   +0.09%     
==========================================
  Files          69       69              
  Lines        6173     6234      +61     
==========================================
+ Hits         5147     5204      +57     
- Misses        669      671       +2     
- Partials      357      359       +2     

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[jskeet]

@zhumin8: I'm definitely expecting to want to change this and the generator code together when your work on transports is done, but I'd like to get what I've got merged (as it does more than just transports) rather than have two moving targets to balance at the same time, if that's okay.