gh-102475: Fix os.path.realpath() for names which look like a drive (GH-155399)

The unresolved part of the path was joined with the resolved part using
join(), so a component containing a colon (e.g. "spam:eggs") reset the path.
It is now simply appended.

A path relative to a drive which does not exist (e.g. "Z:spam") is now
resolved against the root directory of that drive, as the Windows path
normalization does.