Move common Prolog predicates to 'gerrit' package

Because user code will need to import our common rules, move them
to a shorter name "gerrit". This is not likely to collide with any
classes in a running server as we will eventually narrow down the
set of classes available to the PrologEnvironment to a smaller set
than the entire running server, and we aren't likely to ever use a
3rd party library from the gerrit.com domain.

Change-Id: I8f111ea2cb58229159d3a72c081a3808cc6c3438