title: Make pip download prefer to download source-distributions (not wheels)
body: <p>This question <a href="https://stackoverflow.com/questions/7300321/how-to-use-pythons-pip-to-download-and-keep-the-zipped-files-for-a-package">explains</a> how to make pip download and save packages. If I follow this formula, Pip will download wheel (.whl) files if available. </p>
  
  <pre><code>(venv) [user@host glances]$ pip download -d wheelhouse -r build_requirements.txt
  Collecting wheel (from -r build_requirements.txt (line 1))
    File was already downloaded /usr_data/tmp/glances/wheelhouse/wheel-0.29.0-py2.py3-none-any.whl
  Collecting pex (from -r build_requirements.txt (line 2))
    File was already downloaded /usr_data/tmp/glances/wheelhouse/pex-1.1.18-py2.py3-none-any.whl
  Collecting requests (from -r build_requirements.txt (line 3))
    File was already downloaded /usr_data/tmp/glances/wheelhouse/requests-2.12.4-py2.py3-none-any.whl
  Collecting pip (from -r build_requirements.txt (line 4))
    File was already downloaded /usr_data/tmp/glances/wheelhouse/pip-9.0.1-py2.py3-none-any.whl
  Collecting setuptools (from -r build_requirements.txt (line 5))
    File was already downloaded /usr_data/tmp/glances/wheelhouse/setuptools-32.3.1-py2.py3-none-any.whl
  Successfully downloaded wheel pex requests pip setuptools
  </code></pre>
  
  <p>Every single file that it downloaded was a Wheel - but what if I want to get a different kind of file?</p>
  
  <p>I actually want to download the sdist (.tar.gz) files in preference to .whl files? Is there a way to tell Pip what kinds of files I actually want it to get? So instead of getting a directory full of wheels I might want a bunch of tar.gz files.</p>