added prefer_simple_messages feature gate

Author: KevinRK29

mypy/semanal.py

@@ -7476,7 +7476,8 @@ def name_not_defined(self, name: str, ctx: Context, namespace: str | None = None
             return
         message = f'Name "{name}" is not defined'
         if (
-            "." not in name
+            not self.msg.prefer_simple_messages()
+            and "." not in name
             and not (name.startswith("__") and name.endswith("__"))
             and f"builtins.{name}" not in SUGGESTED_TEST_FIXTURES
         ):