CPack/{DEB,RPM}: Append suffix to CPACK_{DEBIAN,RPM}_FILE_NAME if missing

Previously we issued an error when the .rpm suffix is missing. Instead, append the suffix automatically. This matches the behavior of CPACK_ARCHIVE_FILE_NAME, to which the archive format suffix is automatically appended. With this change, developers can simply do

set(CPACK_RPM_comp_FILE_NAME "${CPACK_ARCHIVE_comp_FILE_NAME}")

Topic-rename: cpack-auto-suffixes