Auto merge of #29754 - mrobinson:organize-python-by-platform, r=mukilan

Start organizing platform-specific Python code

This starts to split platform-specific Python code into its own module, which should help to tidy up our mach commands and make things more reusable. This is step one toward fixing #25335.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they just change mach scripts.

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