Simplify G level metadata handling

The original fix from MNG-7055 can be tremendously simplified: the plugins about to be deployed are JARs with embedded plugin descriptor, that contains all the required information (GA, prefix, name) to make G level metadata generator does everything on it's own, no need for hoops and loops at all. Actually, with this, all 3 generators becomes self-sufficient, and no need for caller to even fiddle with deploying metadata (as it happens automatically, courtesy of resolver and resolver-provider in maven).