Cached ARCore-style nested static ObjC xcframeworks now build correctly

Projects caching ARCore and similar Google SDK static Objective-C xcframeworks (layout Headers/<Module>/ with prefix-self-importing headers) can now build and link them through the binary cache behind a dynamic framework. The mapper consumes nested layouts through the xcframework’s own module map, with the Headers root on the search path, so the module is defined exactly once and prefixed imports like <Module/Header.h> resolve. A flat-headers path stays untouched. Validated end-to-end against a fixture mirroring ARCoreGARSession and ARCoreGeospatial.