Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/@python2/ast.pyi

@@ -0,0 +1,28 @@
+# Python 2.7 ast
+
+# Rename typing to _typing, as not to conflict with typing imported
+# from _ast below when loaded in an unorthodox way by the Dropbox
+# internal Bazel integration.
+import typing as _typing
+from typing import Any, Iterator, Optional, Union
+
+from _ast import *
+from _ast import AST, Module
+
+def parse(source: Union[str, unicode], filename: Union[str, unicode] = ..., mode: Union[str, unicode] = ...) -> Module: ...
+def copy_location(new_node: AST, old_node: AST) -> AST: ...
+def dump(node: AST, annotate_fields: bool = ..., include_attributes: bool = ...) -> str: ...
+def fix_missing_locations(node: AST) -> AST: ...
+def get_docstring(node: AST, clean: bool = ...) -> str: ...
+def increment_lineno(node: AST, n: int = ...) -> AST: ...
+def iter_child_nodes(node: AST) -> Iterator[AST]: ...
+def iter_fields(node: AST) -> Iterator[_typing.Tuple[str, Any]]: ...
+def literal_eval(node_or_string: Union[str, unicode, AST]) -> Any: ...
+def walk(node: AST) -> Iterator[AST]: ...
+
+class NodeVisitor:
+    def visit(self, node: AST) -> Any: ...
+    def generic_visit(self, node: AST) -> Any: ...
+
+class NodeTransformer(NodeVisitor):
+    def generic_visit(self, node: AST) -> Optional[AST]: ...