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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---
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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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python/servo/testing_commands.py

@@ -40,7 +40,7 @@ from servo.command_base import (
     call, check_call, check_output,
 )
 from servo_tidy_tests import test_tidy
-from servo.util import host_triple
+from servo.platform import host_triple
 
 SCRIPT_PATH = os.path.split(__file__)[0]
 PROJECT_TOPLEVEL_PATH = os.path.abspath(os.path.join(SCRIPT_PATH, "..", ".."))