Zuul has transitioned zuul.projects to a dict that mirrors
zuul._projects. Convert users back to zuul.projects so we can remove
the underscore version in zuul.
Change-Id: Ib71241a862cf29f6e542dc1d4a2c5cd554d5d7d8
Depends-On: I3c011f72933e98ccbf8badf0e9197c8659766c51