Add presingletons

PreSingletons use the same interface as Singletons, but run after
parsing all the blueprint files but before running any mutators
or GenerateBuildActions.  They can be used to perform global setup,
including generating rules, that may be referenced by later
mutator or generate passes.

Test: m checkbuild
Change-Id: I4b93425a724e739f5b8843262efb8804b3bf3531