Move contents of builtins/* to stdlib/*. This simplifies finding stubs.

stdlib/3/gc.pyi

@@ -0,0 +1,10 @@
+# Stubs for gc
+
+# NOTE: These are incomplete!
+
+import typing
+
+def collect(generation: int = ...) -> int: ...
+def disable() -> None: ...
+def enable() -> None: ...
+def isenabled() -> bool: ...