Cached ARCore-style nested static Objective-C xcframeworks now build correctly

Static Objective-C xcframeworks that keep their module map and headers in a Headers// subdirectory, such as Google’s ARCore, now compile correctly when consumed through the binary cache behind a dynamic framework. The mapper distinguishes nested layouts from flat ones and uses the xcframework’s own module map with the Headers root on the search path, avoiding both missing-header and shadowed-module errors.