Function find_mountpoint::find_mountpoint_pre_canonicalized

pub fn find_mountpoint_pre_canonicalized(path: &Path) -> Result<&Path, Error>

Find the prefix (drive letter or UNC path / sharepoint) for the (already canonicalized) provided paths.

This version relies on Windows-only PrefixComponents but is safe. Which is good, because it's completely untested.

It presumes that you will be passing in fully-qualified canonical paths.