make py lib a self-contained directory again

- move and merge _py/ bits back to py/
- fixes all around

--HG--
branch : trunk

testing/pytest/test_parseopt.py

@@ -1,5 +1,5 @@
 import py
-from _py.test import parseopt
+from py.impl.test import parseopt
 
 class TestParser:
     def test_init(self, capsys):