Cached static Objective-C xcframeworks with nested module map headers now build through their own module map

Cached static Objective-C xcframeworks whose module map and headers live inside a Headers/<Module>/ subdirectory and cross-import with the <Module/Header> prefix (such as Google’s ARCore) now consume their own module map through -fmodule-map-file, with the xcframework’s Headers root on HEADER_SEARCH_PATHS. Flat-layout handling is unchanged, and the module is defined exactly once.