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.
replace uniloadDir hack with BuiltUniloadCatalog

this only affects built meteor for now
don't run git if there's a colon
Move uniload package list to uniload.js

Actually verify that uniloaded packages are in the list. Add missing
'ejson'. Remove (ah well) test that relies on ability to uniload an app
package (which shouldn't work anyway, but it would be nice to test
uniload Assets...)
uniload from checkout uses separate catalog
Use official catalog for springboarding

Read it into memory at initialize time
avoid reentrant complete refreshes
versions update