Remove duplicate names from posixpath.pyi. (#1468)

posixpath.pyi did a '*' import of genericpath.pyi and then redefined
everything it imported. Removes the duplicate definitions from posixpath
and improves the ones in genericpath.

stdlib/2/genericpath.pyi

@@ -1,10 +1,12 @@
+from typing import AnyStr, List
+
 class _unicode: ...
 
-def exists(path): ...
-def isfile(path): ...
-def isdir(s): ...
-def getsize(filename): ...
-def getmtime(filename): ...
-def getatime(filename): ...
-def getctime(filename): ...
-def commonprefix(m): ...
+def commonprefix(list: List[AnyStr]) -> AnyStr: ...
+def exists(path: unicode) -> bool: ...
+def getatime(path: unicode) -> float: ...
+def getmtime(path: unicode) -> float: ...
+def getctime(path: unicode) -> float: ...
+def getsize(path: unicode) -> int: ...
+def isfile(path: unicode) -> bool: ...
+def isdir(path: unicode) -> bool: ...