Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/site.pyi

@@ -0,0 +1,15 @@
+import sys
+from typing import Iterable, List, Optional
+
+PREFIXES: List[str]
+ENABLE_USER_SITE: Optional[bool]
+USER_SITE: Optional[str]
+USER_BASE: Optional[str]
+
+if sys.version_info < (3,):
+    def main() -> None: ...
+
+def addsitedir(sitedir: str, known_paths: Optional[Iterable[str]] = ...) -> None: ...
+def getsitepackages(prefixes: Optional[Iterable[str]] = ...) -> List[str]: ...
+def getuserbase() -> str: ...
+def getusersitepackages() -> str: ...