Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/ctypes/util.pyi

@@ -0,0 +1,7 @@
+import sys
+from typing import Optional
+
+def find_library(name: str) -> Optional[str]: ...
+
+if sys.platform == "win32":
+    def find_msvcrt() -> Optional[str]: ...