core/unit: do not use unit path cache in unit_need_daemon_reload()

core/unit: do not use unit path cache in unit_need_daemon_reload()

When unit_need_daemon_reload() calls unit_find_dropin_paths() to check
for new drop-in configs, the manager's unit path cache is used to limit
which directories are considered. If a new drop-in directory is created,
it may not be in the unit path cache, and hence unit_need_daemon_reload()
may return false, despite a new drop-in being present. However, if a
unit path cache is not given to unit_file_find_dropin_paths() at all,
then it behaves as if the target path was found in the unit path cache.

So, to fix this, adapt unit_find_dropin_paths() to take a boolean
argument indicating whether or not to pass along the unit path cache.
Set this to false in unit_need_daemon_reload().

Fixes #31752