Deprecate ResourceLoaderGetStartupModules

In bug 63240, Roan calls it a 'misfeature', which I think is exactly right. It
has been used as a lazy workaround for having to think about load-order,
dependency management, and race conditions.

Bug: 63240
Change-Id: Ic48ad39c6d3d166d6bb8f60dfdd5f95aa7ed16a6
(cherry picked from commit e8b46e5e6660c9fbce01c982e98c7662b135a130)