Static Objective-C xcframeworks (ARCore) build correctly from the cache

Static Objective-C xcframeworks - including Google’s ARCore shapes with nested Headers// subdirectories where headers cross-import with <ModuleName/Header.h> - now compile through the binary cache. The mapper distinguishes nested and flat layouts: nested ones are consumed via the xcframework’s own module map with the Headers root on the search path; flat ones keep their derived prefix-stripped umbrella. Both the “module redefinition” symptom from rc.1 and the “import of shadowed module” symptom from rc.3 are resolved.