Fix --package-root tests for Windows and Python 3.13+

[emmatyping]

This PR should fix the rest of the test failures in #19545.

A change to os.path.relpath in 3.13 seems to have broken the handling of Windows paths beginning with \\. To resolve this issue, we don't split the drive letter off of the path and instead verify the path is on the current drive. If it isn't it will never resolve to the package root because that must be on the same drive as the CWD:

mypy/mypy/main.py

Lines 1571 to 1572 in 5b03024

	if drive and drive != current_drive:
	parser.error(f"Package root must be on current drive: {drive + root!r}")

Keeping the drive letter allows relpath to properly generate a relative path and make the tests pass.

I need to investigate if the relpath change is a regression in CPython.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ilevkivskyi]

Thanks! Let me try this.