Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/site.pyi

@@ -0,0 +1,12 @@
+from typing import Iterable, List, Optional
+
+PREFIXES: List[str]
+ENABLE_USER_SITE: Optional[bool]
+USER_SITE: Optional[str]
+USER_BASE: Optional[str]
+
+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: ...