pkg: pkg_verify_dependency(): add return error flags pointer. presently, three error flags are defined: PKG_ERRF_OK: everything went fine (no error) PKG_ERRF_PACKAGE_NOT_FOUND: a graph node couldn't be verified because there was no database entry for it PKG_ERRF_PACKAGE_VER_MISMATCH: a graph node couldn't be verified because the entry in the database did not match versioning constraints