Move DEFAULT_TRACK to catalog module

It's a constant, so no need to have to think "hmm, I wonder if I should
be writing catalog.official.DEFAULT_TRACK or
catalog.complete.DEFAULT_TRACK?"
Move towards non-singleton catalog.complete

We're going to make uniload use a different flavor of "complete" catalog
soon.  So we need to reduce the number of singleton-ish references to
it.

Also, we need one PackageCache per catalog, so stop it from being a
singleton too.