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/log/test_log.py

@@ -1,7 +1,7 @@
 import py
 import sys
 
-from py.__.log.log import default_keywordmapper
+from _py.log.log import default_keywordmapper
 
 callcapture = py.io.StdCapture.call