Fix ARCore and Google SDK static Objective-C xcframework caching

Several fixes restore builds for ARCore (and similarly-shaped Google SDK) static Objective-C xcframeworks consumed through the module cache. Module-map-bearing xcframeworks no longer have their Headers directory added twice to HEADER_SEARCH_PATHS, which removes the redefinition of module 'X' error introduced alongside library product caching. Cached static Objective-C xcframeworks consumed behind a dynamic framework now get the xcframework’s Headers root on the search path and, for nested layouts where the module map sits inside Headers/<ModuleName>/, the xcframework’s own module map is used so the module is defined exactly once and the prefixed imports resolve. Flat layouts (e.g. GoogleMaps) keep their original, deduplicated behavior.