Force fedmsg module name to 'pkgdb'.

By default, fedmsg tries to guess the module name by inspecting the call
stack.  For pkgdb2, it would default to 'pkgdb2'.

The pkgdb2 messages are, from what I can tell, generally backwards
compatible with old pkgdb1 messages *except* for the topic.  This change
should (barring any other things) make them fully backwards compatible.