fix setup-eggs for :develop in 2.5.2 nexedi
This MR is intent to fix setup-eggs usage for zc.recipe.egg:develop.
This first commit it just to fix the develop itself, however the implementation of zc.buildout.easy_install.develop is different from zc.buildout.easy_install.build and it don't consider PYTHONEXTRAPATH to extend the PYTHONPATH currently.