Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_posixsubprocess.pyi

@@ -0,0 +1,24 @@
+# NOTE: These are incomplete!
+
+from typing import Callable, Sequence, Tuple
+
+def cloexec_pipe() -> Tuple[int, int]: ...
+def fork_exec(
+    args: Sequence[str],
+    executable_list: Sequence[bytes],
+    close_fds: bool,
+    fds_to_keep: Sequence[int],
+    cwd: str,
+    env_list: Sequence[bytes],
+    p2cread: int,
+    p2cwrite: int,
+    c2pred: int,
+    c2pwrite: int,
+    errread: int,
+    errwrite: int,
+    errpipe_read: int,
+    errpipe_write: int,
+    restore_signals: int,
+    start_new_session: int,
+    preexec_fn: Callable[[], None],
+) -> int: ...