Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/gc.pyi

@@ -0,0 +1,40 @@
+import sys
+from typing import Any, Dict, List, Optional, Tuple
+
+DEBUG_COLLECTABLE: int
+DEBUG_LEAK: int
+DEBUG_SAVEALL: int
+DEBUG_STATS: int
+DEBUG_UNCOLLECTABLE: int
+callbacks: List[Any]
+garbage: List[Any]
+
+def collect(generation: int = ...) -> int: ...
+def disable() -> None: ...
+def enable() -> None: ...
+def get_count() -> Tuple[int, int, int]: ...
+def get_debug() -> int: ...
+
+if sys.version_info >= (3, 8):
+    def get_objects(generation: Optional[int] = ...) -> List[Any]: ...
+
+else:
+    def get_objects() -> List[Any]: ...
+
+if sys.version_info >= (3, 7):
+    def freeze() -> None: ...
+    def unfreeze() -> None: ...
+    def get_freeze_count() -> int: ...
+
+def get_referents(*objs: Any) -> List[Any]: ...
+def get_referrers(*objs: Any) -> List[Any]: ...
+def get_stats() -> List[Dict[str, Any]]: ...
+def get_threshold() -> Tuple[int, int, int]: ...
+def is_tracked(__obj: Any) -> bool: ...
+
+if sys.version_info >= (3, 9):
+    def is_finalized(__obj: Any) -> bool: ...
+
+def isenabled() -> bool: ...
+def set_debug(__flags: int) -> None: ...
+def set_threshold(threshold0: int, threshold1: int = ..., threshold2: int = ...) -> None: ...