title: Make Python libraries installed with requirements.txt immediately available
body: <p>Here's a very simple requirements.txt:</p>
  
  <pre><code>numpy
  sharedmem
  </code></pre>
  
  <p>The problem is, sharedmem actually <a href="https://github.com/rainwoodman/sharedmem/blob/master/setup.py#L1" rel="nofollow noreferrer">depends on having numpy available to even run its setup.py</a>, it's an install dependency. Running <code>pip install -r requirements.txt</code> does try to install numpy first, as expected, but sharedmem still breaks on trying to import <code>numpy.distutils.core</code>. Presumably because pip installs libraries into a temporary build directory first and it's not in the import path.</p>
  
  <p>Any ideas on how to deal with it?</p>
  
  <p>P.S. Looks like this is a dupe: <a href="https://stackoverflow.com/questions/37063522/in-python-pip-r-requirements-txt-doesnt-install-packages-recursively">In Python, `pip -r requirements.txt` doesn&#39;t install packages *recursively*?</a>, but the problem wasn't resolved there either.</p>