core: reload cache if it's dirty when starting a UNIT_NOT_FOUND unit

The time-based cache allows starting a new unit without an expensive
daemon-reload, unless there was already a reference to it because of
a dependency or ordering from another unit.
If the cache is out of date, check again if we can load the
fragment.

src/shared/unit-file.c

@@ -199,7 +199,7 @@ static bool lookup_paths_mtime_exclude(const LookupPaths *lp, const char *path)
                streq_ptr(path, lp->runtime_control);
 }
 
-static bool lookup_paths_mtime_good(const LookupPaths *lp, usec_t mtime) {
+bool lookup_paths_mtime_good(const LookupPaths *lp, usec_t mtime) {
         char **dir;
 
         STRV_FOREACH(dir, (char**) lp->search_path) {