make py lib a self-contained directory again

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

--HG--
branch : trunk

testing/root/test_com.py

@@ -1,7 +1,7 @@
 
 import py
 import os
-from _py._com import Registry, MultiCall, HookRelay, varnames
+from py.impl._com import Registry, MultiCall, HookRelay, varnames
 
 def test_varnames():
     def f(x):

testing/root/test_xmlgen.py

@@ -1,6 +1,6 @@
 
 import py
-from _py.xmlgen import unicode, html
+from py.impl.xmlgen import unicode, html
 
 class ns(py.xml.Namespace): 
     pass