Allow -fmodules in direct depend mode (#482)

Allow -fmodules in direct depend mode (#482)

Clang creates the internal on-disk representation of modules
in the background on demand as necessary. This means that ccache does
not need to cache that, all that it needs to cache is the actual
compilation result and the dependencies. Which in the case of modules
includes also the module.modulemap files, and Clang outputs such
dependencies correctly only in the depend mode, in the preprocessed
output there is no way to find out the module dependencies.

Therefore support -fmodules in direct depend mode by more or less
ignoring it. Tested with source build of LLVM/Clang configured
with -DLLVM_ENABLE_MODULES=On (and the build actually occassionally
fails because of module problems, but that happens regardless of ccache).

See also pull request #130 (and this reuses the tests from there).