Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_warnings.pyi

@@ -0,0 +1,67 @@
+import sys
+from typing import Any, Dict, List, Optional, Tuple, Type, Union, overload
+
+if sys.version_info >= (3, 0):
+    _defaultaction: str
+    _onceregistry: Dict[Any, Any]
+else:
+    default_action: str
+    once_registry: Dict[Any, Any]
+
+filters: List[Tuple[Any, ...]]
+
+if sys.version_info >= (3, 6):
+    @overload
+    def warn(
+        message: str, category: Optional[Type[Warning]] = ..., stacklevel: int = ..., source: Optional[Any] = ...
+    ) -> None: ...
+    @overload
+    def warn(message: Warning, category: Any = ..., stacklevel: int = ..., source: Optional[Any] = ...) -> None: ...
+    @overload
+    def warn_explicit(
+        message: str,
+        category: Type[Warning],
+        filename: str,
+        lineno: int,
+        module: Optional[str] = ...,
+        registry: Optional[Dict[Union[str, Tuple[str, Type[Warning], int]], int]] = ...,
+        module_globals: Optional[Dict[str, Any]] = ...,
+        source: Optional[Any] = ...,
+    ) -> None: ...
+    @overload
+    def warn_explicit(
+        message: Warning,
+        category: Any,
+        filename: str,
+        lineno: int,
+        module: Optional[str] = ...,
+        registry: Optional[Dict[Union[str, Tuple[str, Type[Warning], int]], int]] = ...,
+        module_globals: Optional[Dict[str, Any]] = ...,
+        source: Optional[Any] = ...,
+    ) -> None: ...
+
+else:
+    @overload
+    def warn(message: str, category: Optional[Type[Warning]] = ..., stacklevel: int = ...) -> None: ...
+    @overload
+    def warn(message: Warning, category: Any = ..., stacklevel: int = ...) -> None: ...
+    @overload
+    def warn_explicit(
+        message: str,
+        category: Type[Warning],
+        filename: str,
+        lineno: int,
+        module: Optional[str] = ...,
+        registry: Optional[Dict[Union[str, Tuple[str, Type[Warning], int]], int]] = ...,
+        module_globals: Optional[Dict[str, Any]] = ...,
+    ) -> None: ...
+    @overload
+    def warn_explicit(
+        message: Warning,
+        category: Any,
+        filename: str,
+        lineno: int,
+        module: Optional[str] = ...,
+        registry: Optional[Dict[Union[str, Tuple[str, Type[Warning], int]], int]] = ...,
+        module_globals: Optional[Dict[str, Any]] = ...,
+    ) -> None: ...