[clang-scan-deps] ignore top-level module dependencies that aren't actually imported

Whenever -fmodule-name=top_level_module name is parsed, and clang actually tries to
import top_level_module, the headers are imported textually and the module isn't actually
built. However, the dependency scanner could still record it as a potential dependency
if the module was reimported and thus recorded by the preprocessor callbacks.
This change avoids collecting this kind of module as a dependency by verifying that we don't
collect top level modules without actual PCM files.

Differential Revision: https://reviews.llvm.org/D106100