Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/sysconfig.pyi

@@ -0,0 +1,17 @@
+from typing import IO, Any, Dict, List, Optional, Tuple, overload
+
+def get_config_var(name: str) -> Optional[str]: ...
+@overload
+def get_config_vars() -> Dict[str, Any]: ...
+@overload
+def get_config_vars(arg: str, *args: str) -> List[Any]: ...
+def get_scheme_names() -> Tuple[str, ...]: ...
+def get_path_names() -> Tuple[str, ...]: ...
+def get_path(name: str, scheme: str = ..., vars: Optional[Dict[str, Any]] = ..., expand: bool = ...) -> Optional[str]: ...
+def get_paths(scheme: str = ..., vars: Optional[Dict[str, Any]] = ..., expand: bool = ...) -> Dict[str, str]: ...
+def get_python_version() -> str: ...
+def get_platform() -> str: ...
+def is_python_build(check_home: bool = ...) -> bool: ...
+def parse_config_h(fp: IO[Any], vars: Optional[Dict[str, Any]] = ...) -> Dict[str, Any]: ...
+def get_config_h_filename() -> str: ...
+def get_makefile_filename() -> str: ...