[Modules] Move modulemaps to header search directories. NFC intended.

In code we use `#include "llvm/Lib/Header.h"` which is located in
"llvm/include/llvm/Lib/Header.h", so we use "llvm/include/" as a header
search path. We should put modulemaps in the same directory and
shouldn't rely on clang to search in immediate subdirectories.

rdar://106677321

Differential Revision: https://reviews.llvm.org/D148776