Reimplement AddMissingDependencies

AddMissingDependencies is needed in BaseModuleContext in order
to allow PathsForModuleSrc and friends to be called in mutators,
It's not a simple move, as it currently writes to a module
field that would be lost if any mutators cloned new variants
by calling CreateVariations.  Reimplement it using a mutated
property instead.

Test: m checkbuild
Change-Id: I851125065e4c5302b552773dae4640426c62965e