When looking for a directory name that matches the extension UUID,
we can just as well use GLib's dirname()/basename() functions
instead of wrapping the path in a GFile.
We also know that the original path corresponds to a regular file
and not a directory, so rearrange the loop to avoid a lookup that
is guaranteed to fail.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2832>
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