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_pytester.py

@@ -1,5 +1,5 @@
 import py
-from py.__.test.plugin.pytest_pytester import LineMatcher, LineComp
+from _py.test.plugin.pytest_pytester import LineMatcher, LineComp
 
 def test_reportrecorder(testdir):
     registry = py._com.Registry()