Fix extractor watchdog when --domain-ontology receives a full path

If the --domain-ontology argument gave the full path to the domain rule, we would previously do the wrong thing, leading to errors like this:

(tracker-miner-fs-3:2): Tracker-DEBUG: 12:57:48.820: Setting up watch on tracker-extract at /app/share/tracker3/domain-ontologies/org.gnome.Music.Devel.domain.rule.Tracker3.Miner.Extract (autostart: yes)

(tracker-miner-fs-3:2): GLib-GIO-CRITICAL **: 12:57:48.820: g_bus_watch_name: assertion 'g_dbus_is_name (name)' failed

Now we correctly get the name from the domain rule file.

Needed for gnome-music!732 (closed)