Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/genericpath.pyi

@@ -0,0 +1,27 @@
+import sys
+from typing import AnyStr, Sequence, Text, Union
+
+if sys.version_info >= (3, 0):
+    def commonprefix(m: Sequence[str]) -> str: ...
+
+else:
+    def commonprefix(m: Sequence[AnyStr]) -> AnyStr: ...
+
+if sys.version_info >= (3, 6):
+    from builtins import _PathLike
+    def exists(path: Union[AnyStr, _PathLike[AnyStr]]) -> bool: ...
+
+else:
+    def exists(path: Text) -> bool: ...
+
+def isfile(path: Text) -> bool: ...
+def isdir(s: Text) -> bool: ...
+def getsize(filename: Text) -> int: ...
+def getmtime(filename: Text) -> float: ...
+def getatime(filename: Text) -> float: ...
+def getctime(filename: Text) -> float: ...
+
+if sys.version_info >= (3, 4):
+    def samestat(s1: str, s2: str) -> int: ...
+    def samefile(f1: str, f2: str) -> int: ...
+    def sameopenfile(fp1: str, fp2: str) -> int: ...