Change PathLikeWithPosition<P> into a non-generic type and replace ad-hoc Windows path parsing (#15373)

This simplifies `PathWithPosition` by making the common use case
concrete and removing the manual, incomplete Windows path parsing.
Windows paths also don't get '/'s replaced by '\\'s anymore to limit the
responsibility of the code to just parsing out the suffix and creating
`PathBuf` from the rest. `Path::file_name()` is now used to extract the
filename and potential suffix instead of manual parsing from the full
input. This way e.g. Windows paths that begin with a drive letter are
handled correctly without platform-specific hacks.

Release Notes:

- N/A