title: DistributionNotFound when trying to install ZRS
body: <p>I have a site with Plone 4.2.5 in which I want to install ZRS.</p>
  
  <p>I added the <code>[zrs]</code> extra to the <code>zeoserver</code> part:</p>
  
  <pre><code>[zeoserver]
  recipe = plone.recipe.zeoserver[zrs]
  </code></pre>
  
  <p>I also pinned the right versions of the recipes and packages involved:</p>
  
  <pre><code>[versions]
  plone.recipe.zeoserver = 1.2.8
  plone.recipe.zope2instance = 4.2.18
  Twisted = 15.4.0
  zc.zrs = 2.4.4
  </code></pre>
  
  <p>But, when I run buildout, I get the following error:</p>
  
  <pre><code>$ bin/buildout
  ...
  While:
    Installing.
  
  An internal error occurred due to a bug in either zc.buildout or in a
  recipe being used:
  Traceback (most recent call last):
    File "/home/plone/my.project/eggs/zc.buildout-2.4.3-py2.7.egg/zc/buildout/buildout.py", line 1992, in main
      getattr(buildout, command)(args)
    File "/home/plone/my.project/eggs/zc.buildout-2.4.3-py2.7.egg/zc/buildout/buildout.py", line 565, in install
      self._compute_part_signatures(install_parts)
    File "/home/plone/my.project/eggs/zc.buildout-2.4.3-py2.7.egg/zc/buildout/buildout.py", line 810, in _compute_part_signatures
      sig = _dists_sig(pkg_resources.working_set.resolve([req]))
    File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 839, in resolve
      raise DistributionNotFound(req, requirers)
  DistributionNotFound: The 'zc.zrs' distribution was not found and is required by plone.recipe.zeoserver
  </code></pre>
  
  <p>The exact same thing runs fine in Plone 4.3.</p>
  
  <p>I'm using the following versions also:</p>
  
  <pre><code>[versions]
  setuptools = 18.3.2
  zc.buildout = 2.4.3
  </code></pre>
  
  <p>Any hint?</p>