Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_imp.pyi

@@ -0,0 +1,17 @@
+import types
+from importlib.machinery import ModuleSpec
+from typing import Any, List
+
+def create_builtin(__spec: ModuleSpec) -> types.ModuleType: ...
+def create_dynamic(__spec: ModuleSpec, __file: Any = ...) -> None: ...
+def acquire_lock() -> None: ...
+def exec_builtin(__mod: types.ModuleType) -> int: ...
+def exec_dynamic(__mod: types.ModuleType) -> int: ...
+def extension_suffixes() -> List[str]: ...
+def get_frozen_object(__name: str) -> types.CodeType: ...
+def init_frozen(__name: str) -> types.ModuleType: ...
+def is_builtin(__name: str) -> int: ...
+def is_frozen(__name: str) -> bool: ...
+def is_frozen_package(__name: str) -> bool: ...
+def lock_held() -> bool: ...
+def release_lock() -> None: ...