Add an accessor to PluginType for the manager service name

There is a method getPluginManager(), but that gets me a the service itself.

What I want is the name of the service.

(The reason for this is that Plugin module is using IDs for plugin types that don't cleanly match up to anything else. So to deal with both plugin types which are declared to Plugin, and those which are not, I need the service ID to match Plugin data's to data detected from plugin manager services.)