[clang][deps] Account for transitive spurious dependencies

In D106100, we started guarding against spurious dependencies on modules that ended up being textual includes and thus didn't have any AST file associated. That patch accounted only for direct dependencies. There's a way how to get spurious dependencies for modules that are transitive. This patch guards against that scenario and adds a test case.

(Note that since D142167, we don't allow `@import FW_Private` with `-fmodule-name=FW` anymore. However, that check lives in sema, which the scanner doesn't run. Being defensive in this patch therefore still makes sense.)

rdar://104324602

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D142165