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/linecache.pyi

@@ -0,0 +1,13 @@
+import sys
+from typing import Any, Dict, List, Optional, Text
+
+_ModuleGlobals = Dict[str, Any]
+
+def getline(filename: Text, lineno: int, module_globals: Optional[_ModuleGlobals] = ...) -> str: ...
+def clearcache() -> None: ...
+def getlines(filename: Text, module_globals: Optional[_ModuleGlobals] = ...) -> List[str]: ...
+def checkcache(filename: Optional[Text] = ...) -> None: ...
+def updatecache(filename: Text, module_globals: Optional[_ModuleGlobals] = ...) -> List[str]: ...
+
+if sys.version_info >= (3, 5):
+    def lazycache(filename: Text, module_globals: _ModuleGlobals) -> bool: ...