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/path/test_cacheutil.py

@@ -1,5 +1,5 @@
 import py
-from py.__.path import cacheutil 
+from _py.path import cacheutil 
 
 class BasicCacheAPITest:
     cache = None