Module-map xcframework headers excluded from header search paths

Targets that depend on a precompiled xcframework whose slice ships an included module.modulemap no longer fail to compile with “Redefinition of module ‘X’”. The generated header search paths no longer add a second copy of those headers, because Xcode’s ProcessXCFramework already extracts them to $(BUILT_PRODUCTS_DIR)/include. xcframeworks without a module map (such as cached Swift/library products) keep their headers on the search path.