Nested static-objc xcframeworks consumed via their own module map (ARCore)

Static Objective-C xcframeworks with nested public headers (headers inside Headers// that re-import siblings as <ModuleName/Sibling.h>) are now consumed directly through the xcframework’s own module map with the Headers root on the search path. The module is defined exactly once and the prefixed self-imports resolve without a derived module map or umbrella rewrite. Flat-layout handling is unchanged. This unblocks caching ARCore-style static ObjC libraries through the module cache.