fix: games tools write whole files, name options

The games importers wrote through fixed .part names, the new options had
no help text, and romfolders stopped on the first setup the offline cache
lacked. RetroArch's rom folder map is regenerated from its source spec
(path null, as the schema says). The installer's Python 3.8 job imports
the games tests without pyyaml, so the region rules load lazily.

scripts/generate_games_index.py

@@ -587,7 +587,8 @@ def print_coverage(coverage: dict, files: dict[Path, str]) -> None:
 
 def main() -> int:
     parser = argparse.ArgumentParser(allow_abbrev=False, description=__doc__.splitlines()[0])
-    parser.add_argument("--output-dir", type=Path, default=OUTPUT)
+    parser.add_argument("--output-dir", type=Path, default=OUTPUT,
+                        help=f"where the index is written (default {OUTPUT})")
     parser.add_argument("--check", action="store_true", help="exit 1 when a file would change")
     parser.add_argument("--quiet", action="store_true", help="print only failures")
     args = parser.parse_args()