[clang][modules][deps] Transitive module maps are not affecting

Currently, the algorithm for gathering affecting module maps includes those defining transitive dependencies. This seems like an over-approximation, since those don't change the semantics of current module build.

(With this patch, `ModulesToProcess` only ever holds modules whose headers will be serialized into the current PCM.)

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D137197