Swap usage of InternalEvents::EventDefinitions.known_event? to Tracking::EventDefinition.find or internal_event_exists?

The last usage of this method is in the internal event matchers.

That also means the matchers currently depend on the metric definitions rather than event definitions. This prevents the matchers from working for internal events which don't have metrics.

We should swap to an matcher implementation better aligning with the functionality of Gitlab::InternalEvents.