Fix destdir_join
Fix destdir_join

The old implementation assumed a path is of Windows iff the second
character is a colon. However, that is not always true.
First, a colon can be included in a non-Windows path. For example, it is
totally fine to have a directory named ':' on Linux 5.17.0 tmpfs.
Second, a Windows path may start with \\ for UNC or extended length.

Use pathlib to handle all of these cases.

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