feat(order_utils.py): schema resolver cache (#1317)

* Implemented basic functionality for using cache layer of LocalRefResolver

* Use `importlib` instead of `imp`, since it's been deprecated. Legacy `load_module()` reloads modules even if they are already imported, causing tests to fail when run in non-deterministic ordering, so we replace it with `import_module()`

python-packages/order_utils/stubs/jsonschema/__init__.pyi

@@ -1,5 +1,11 @@
-from typing import Any, Dict
+from typing import Any, Dict, Tuple
 
-class RefResolver: pass
+
+class RefResolver:
+    def resolve(self, url: str) -> Tuple[str, Dict]:
+        ...
+
+
+class ValidationError(Exception): pass
 
 def validate(instance: Any, schema: Dict, cls=None, *args, **kwargs) -> None: pass