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_funcargs.py

@@ -1,5 +1,5 @@
 import py, sys
-from _py.test import funcargs
+from py.impl.test import funcargs
 
 def test_getfuncargnames():
     def f(): pass