Precompiled xcframeworks no longer redefine their module

Builds against precompiled xcframeworks no longer fail with redefinition of module 'X' when the xcframework ships a module.modulemap. GraphTraverser.librariesPublicHeadersFolders now skips the slice headers for module-map-bearing xcframeworks, since Xcode’s ProcessXCFramework already extracts the matching slice’s module map into $(BUILT_PRODUCTS_DIR)/include (which is on the header search path). Module-map-less xcframeworks (the cached Swift / library products from the module cache) still have their headers added.