Re-organize directory structure (#4971)

See discussion in #2491

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

stubs/mypy-extensions/METADATA.toml

@@ -0,0 +1,2 @@
+version = "0.4"
+python2 = true

stubs/mypy-extensions/mypy_extensions.pyi

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+import abc
+import sys
+from typing import Any, Callable, Dict, Generic, ItemsView, KeysView, Mapping, Optional, Type, TypeVar, Union, ValuesView
+
+_T = TypeVar("_T")
+_U = TypeVar("_U")
+
+# Internal mypy fallback type for all typed dicts (does not exist at runtime)
+class _TypedDict(Mapping[str, object], metaclass=abc.ABCMeta):
+    def copy(self: _T) -> _T: ...
+    # Using NoReturn so that only calls using mypy plugin hook that specialize the signature
+    # can go through.
+    def setdefault(self, k: NoReturn, default: object) -> object: ...
+    # Mypy plugin hook for 'pop' expects that 'default' has a type variable type.
+    def pop(self, k: NoReturn, default: _T = ...) -> object: ...
+    def update(self: _T, __m: _T) -> None: ...
+    if sys.version_info < (3, 0):
+        def has_key(self, k: str) -> bool: ...
+        def viewitems(self) -> ItemsView[str, object]: ...
+        def viewkeys(self) -> KeysView[str]: ...
+        def viewvalues(self) -> ValuesView[object]: ...
+    else:
+        def items(self) -> ItemsView[str, object]: ...
+        def keys(self) -> KeysView[str]: ...
+        def values(self) -> ValuesView[object]: ...
+    def __delitem__(self, k: NoReturn) -> None: ...
+
+def TypedDict(typename: str, fields: Dict[str, Type[_T]], total: bool = ...) -> Type[Dict[str, Any]]: ...
+def Arg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def DefaultArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def NamedArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def DefaultNamedArg(type: _T = ..., name: Optional[str] = ...) -> _T: ...
+def VarArg(type: _T = ...) -> _T: ...
+def KwArg(type: _T = ...) -> _T: ...
+
+# Return type that indicates a function does not return.
+# This type is equivalent to the None type, but the no-op Union is necessary to
+# distinguish the None type from the None value.
+NoReturn = Union[None]  # Deprecated: Use typing.NoReturn instead.
+
+# This is intended as a class decorator, but mypy rejects abstract classes
+# when a Type[_T] is expected, so we can't give it the type we want
+def trait(cls: Any) -> Any: ...
+def mypyc_attr(*attrs: str, **kwattrs: object) -> Callable[[_T], _T]: ...
+
+class FlexibleAlias(Generic[_T, _U]): ...