Cached static-objc-behind-dynamic xcframework headers are on the search path

For cached static Objective-C xcframeworks reached through the binary cache behind a dynamic framework, the consuming target’s HEADER_SEARCH_PATHS now also includes the xcframework slice’s Headers root (filtered by supported platforms). This restores the ability to resolve framework-prefixed sibling imports (#import <Module/Sibling.h>) such as those used in ARCore while keeping the (redefinition of module 'X') regression out of the directly-linked xcframework path.