Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/multiprocessing/spawn.pyi

@@ -0,0 +1,21 @@
+from types import ModuleType
+from typing import Any, Dict, List, Mapping, Optional, Sequence
+
+WINEXE: bool
+WINSERVICE: bool
+
+def set_executable(exe: str) -> None: ...
+def get_executable() -> str: ...
+def is_forking(argv: Sequence[str]) -> bool: ...
+def freeze_support() -> None: ...
+def get_command_line(**kwds: Any) -> List[str]: ...
+def spawn_main(pipe_handle: int, parent_pid: Optional[int] = ..., tracker_fd: Optional[int] = ...) -> None: ...
+
+# undocumented
+def _main(fd: int) -> Any: ...
+def get_preparation_data(name: str) -> Dict[str, Any]: ...
+
+old_main_modules: List[ModuleType]
+
+def prepare(data: Mapping[str, Any]) -> None: ...
+def import_main_path(main_path: str) -> None: ...