UNC path brokenness

[lazka]

Two issues:

The last one fails:

$ mkdir -p /tmp/bla
$ touch /tmp/bla/unctest.py
$ python3 -c "import sys; sys.path.insert(0, r'C:/msys64/tmp/bla'); import unctest"
$ python3 -c "import sys; sys.path.insert(0, r'C:\\msys64\\tmp\\bla'); import unctest"
# broken:
$ python3 -c "import sys; sys.path.insert(0, r'\\\\?\\C:\\msys64\\tmp\\bla'); import unctest"
# works
$ MSYSTEM= python3 -c "import sys; sys.path.insert(0, r'\\\\?\\C:\\msys64\\tmp\\bla'); import unctest"

Maybe related:

$ python3 -c "import os; print(os.path.normpath(r'\\\\?\\C:\\msys64\\tmp\\bla'))"
//?/C:/msys64/tmp/bla

We should not normalize paths starting with with \\?\ even if MSYSTEM is set, as that would break them.

Possible related downstream issues:

• https://github.com/astral-sh/uv/issues/3573#issuecomment-2306726586
• https://github.com/msys2/MINGW-packages/blob/30f86a7a33249b4504d2b12a8ea58769cd4c5f83/mingw-w64-python-setuptools/PKGBUILD#L76-L80