Nested static ObjC xcframeworks build via their own module map

Static Objective-C xcframeworks that lay their headers and module map out in a Headers/<ModuleName>/ subdirectory (ARCore-style, e.g. ARCoreGARSession) now build correctly when consumed through the binary cache behind a dynamic framework. The mapper detects the nested layout and uses the xcframework’s own module map plus the Headers root as the search path, so the module is defined exactly once and the framework-prefixed self-imports like <ARCoreGARSession/GARTrackingState.h> resolve. Flat layouts are unchanged.