systemctl: fix masking of template units

systemctl would try to load the properties of the unit, which is impossible
for template names, and the whole operation would fail. It seems that this
regression was introduced in 00c83b43.

Export the part of unit_find_paths() responsible for locating instance unit
fragments and reuse it from unit_exists() to fix the handling of template
units.

Fixes #6412.

(cherry picked from commit 173471b7)