Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_sitebuiltins.pyi

@@ -0,0 +1,16 @@
+from typing import ClassVar, Iterable, NoReturn, Optional
+from typing_extensions import Literal
+
+class Quitter:
+    name: str
+    eof: str
+    def __init__(self, name: str, eof: str) -> None: ...
+    def __call__(self, code: Optional[int] = ...) -> NoReturn: ...
+
+class _Printer:
+    MAXLINES: ClassVar[Literal[23]]
+    def __init__(self, name: str, data: str, files: Iterable[str] = ..., dirs: Iterable[str] = ...) -> None: ...
+    def __call__(self) -> None: ...
+
+class _Helper:
+    def __call__(self, request: object) -> None: ...