rewrote the initpkg mechanism and moved py lib implementation files to

_py/...  with py/__init__.py containing pointers into them

The new apipkg is only around 70 lines of code and allows
us to get rid of the infamous "py.__." by regular non-magical
"_py." imports. It is also available as a separately installable
package, see http://bitbucket.org/hpk42/apipkg

--HG--
branch : trunk

testing/pytest/plugin/test_pytest_monkeypatch.py

@@ -1,6 +1,6 @@
 import os, sys
 import py
-from py.__.test.plugin.pytest_monkeypatch import MonkeyPatch
+from _py.test.plugin.pytest_monkeypatch import MonkeyPatch
 
 def test_setattr():
     class A: