Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/graphlib.pyi

@@ -0,0 +1,16 @@
+from _typeshed import SupportsItems
+from typing import Generic, Iterable, Optional, Tuple, TypeVar
+
+_T = TypeVar("_T")
+
+class TopologicalSorter(Generic[_T]):
+    def __init__(self, graph: Optional[SupportsItems[_T, Iterable[_T]]] = ...) -> None: ...
+    def add(self, node: _T, *predecessors: _T) -> None: ...
+    def prepare(self) -> None: ...
+    def is_active(self) -> bool: ...
+    def __bool__(self) -> bool: ...
+    def done(self, *nodes: _T) -> None: ...
+    def get_ready(self) -> Tuple[_T, ...]: ...
+    def static_order(self) -> Iterable[_T]: ...
+
+class CycleError(ValueError): ...