Caching static Objective-C and ARCore-style xcframeworks works end to end

A chain of fixes unblocks caching static Objective-C xcframeworks behind dynamic frameworks, in particular ARCore-style Google SDK layouts with nested Headers// directories. Headers/module-map duplicates in the search path are skipped when the xcframework ships a module map, the Headers root is exposed for cached static-objc targets, nested ARCore-style layouts are consumed via their own module map (no redefinition), and static framework product copies no longer ship a second module map. Projects that cache ARCore, ARCore Geospatial, and similarly-shaped frameworks now build successfully through the binary cache.