fix(scripts): add missing defaultdict import in schema validation

`_build_schema_index()` uses `defaultdict` but `collections.defaultdict`
was not imported, causing `NameError` on every rebuild.

Author: ivan-aksamentov

scripts/lib/schema.py

@@ -1,6 +1,7 @@
 import json
 import re
 import urllib.request
+from collections import defaultdict
 from dataclasses import dataclass, field
 from functools import lru_cache
 from pathlib import Path