Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/email/contentmanager.pyi

@@ -0,0 +1,11 @@
+from email.message import Message
+from typing import Any, Callable
+
+class ContentManager:
+    def __init__(self) -> None: ...
+    def get_content(self, msg: Message, *args: Any, **kw: Any) -> Any: ...
+    def set_content(self, msg: Message, obj: Any, *args: Any, **kw: Any) -> Any: ...
+    def add_get_handler(self, key: str, handler: Callable[..., Any]) -> None: ...
+    def add_set_handler(self, typekey: type, handler: Callable[..., Any]) -> None: ...
+
+raw_data_manager: ContentManager