Fix module redefinition when xcframeworks ship their own module map

Targets depending on a precompiled xcframework that ships a module.modulemap now compile correctly. Tuist no longer adds the xcframework slice’s Headers to HEADER_SEARCH_PATHS when the xcframework already provides a module map, since Xcode’s ProcessXCFramework already exposes that module map at $(BUILT_PRODUCTS_DIR)/include. Modules without a module map (such as cached Swift/library products) keep their headers in the search path, so the module-cache library feature is unaffected.