title: pip freeze does not show all installed packages
body: <p>I am using a <code>virtualenv</code>. I have <code>fabric</code> installed, with <code>pip</code>. But a <code>pip freeze</code> does not give any hint about that. The package is there, in my <code>virtualenv</code>, but pip is silent about it. Why could that be? Any way to debug this?</p>