registration: Don't initialize MWServices super early

registration: Don't initialize MWServices super early

Previously, MediaWikiServices would be initialized on the first
wfLoadExtension() call, which is super early and before it should be.
There's actually no reason we need to create the BagOStuff object in the
constructor, so let's defer it to loadFromQueue(). It now gets called
from the top of Setup.php, which is still a little early, but better
than before.

Bug: T154960
Change-Id: I3feef3b974ba1ba3afec0d453e1899cd476e72fb