Bug 1712151: Add test to verify virtualenv compatibility r=ahal

This adds two main compatibility guarantees:
1. Vendored dependencies <=> Pypi-downloaded dependencies
2. Global Mach dependencies <=> command-specific dependencies

As part of this, a new `vendored:` action was added to the virtualenv
definition format. Otherwise similar to `pth:` paths, `vendored:`
packages are assumed to be "pip install"-able.

Some validation (the `.dist-info`/`PKG-INFO` checks) was added to
`requirements.py` to verify that `pth:` and `vendored:` are correctly
used.

Differential Revision: https://phabricator.services.mozilla.com/D122900

python/mach_commands.py

@@ -81,6 +81,7 @@ def python(
         append_env["PYTHONPATH"] = os.pathsep.join(
             os.path.join(command_context.topsrcdir, pth.path)
             for pth in requirements.pth_requirements
+            + requirements.vendored_requirements
         )
     else:
         command_context.virtualenv_manager.ensure()