title: pip -r not working as expected in virtualenv
body: <p>I have a really strange issue with the behavior of pip in a virtualenv.</p>
  
  <p>I have set</p>
  
  <pre><code>export PIP_REQUIRE_VIRTUALENV=true
  export PIP_DOWNLOAD_CACHE=$HOME/.pip/cache
  export VIRTUALENV_DISTRIBUTE=true
  </code></pre>
  
  <p>Now, when I'm in my virtualenv and want to install my dependencies:</p>
  
  <pre><code>pip install -r requirements/_base.txt
  </code></pre>
  
  <p>(for testing purposes, requirements/_base.txt only contains:)</p>
  
  <pre><code>Django==1.5
  </code></pre>
  
  <p>Anyway, when I try to install it, I only get:</p>
  
  <pre><code>Downloading/unpacking Django==1.5 (from -r requirements/_base.txt (line 1))
  Running setup.py egg_info for package Django
  
  warning: no previously-included files matching '__pycache__' found under directory '*'
  warning: no previously-included files matching '*.py[co]' found under directory '*'
  </code></pre>
  
  <p>And after that, Django is not installed.</p>
  
  <p>Here comes the strange part: When I install Django manually with</p>
  
  <pre><code>pip install Django==1.5
  </code></pre>
  
  <p>it's working perfectly (though the same two warnings appear):</p>
  
  <pre><code>Downloading/unpacking Django==1.5
  Running setup.py egg_info for package Django
  
  warning: no previously-included files matching '__pycache__' found under directory '*'
  warning: no previously-included files matching '*.py[co]' found under directory '*'
  Installing collected packages: Django
  Running setup.py install for Django
  changing mode of build/scripts-2.7/django-admin.py from 644 to 755
  
  warning: no previously-included files matching '__pycache__' found under directory '*'
  warning: no previously-included files matching '*.py[co]' found under directory '*'
  changing mode of /usr/local/share/python/django-admin.py to 755
  Successfully installed Django
  Cleaning up...
  </code></pre>