OpMatcherService: improve javadoc
OpMatcherService: add getOpString methods

The OpService can now lean on these methods when generating debug info.
DefaultOpService: add javadoc to assign method
Move findModules method to OpMatcherService, too

This method is really the heart and soul of Op matching, and thus
belongs in the OpMatcherService. And it makes OpService even more
focused on the Ops themselves.