title: preventing addition of system Python path to virtualenv easy-install.pth by python setup.py develop
body: <p>When I attempt to add a Python package under development with <code>distribute</code> to a <code>virtualenv</code> via <code>python setup.py develop</code>, the system Python path (<code>/usr/lib/python2.7/dist-packages</code> on my Ubuntu 13.04 system) is automatically added to the virtualenv's <code>easy-install.pth</code> file. This seems to occur when the package under development has a dependency that is available in the system Python path even if it is also installed and active in the virtualenv. Is there some way to prevent <code>python setup.py develop</code> from adding the system Python path to <code>easy-install.pth</code> when a required package already is installed in the virtualenv? I want to avoid adding the system Python path to <code>easy-install.pth</code> because it confuses the <code>distribute</code> entry point mechanism of Python packages that are available both in the virtualenv and in the system Python path (e.g., ipython). </p>