[clang][deps] Fix modulemap file path for implementation of module

Use the name "as requested" for the path of the implemented module's
modulemap file, just as we do for other modulemap file paths. This fixes
fatal errors with modules where we tried to find framework headers
relative to the wrong directory when imported by an implementation file
of the same module.

rdar://104619123

Differential Revision: https://reviews.llvm.org/D142501