title: pip.main install fails with 'module' object has no attribute 'main'
body: <p>I am trying to install few of the python packages from within a python script and I am using <code>pip.main(install)</code> for that. Below is code snippet</p>
  
  <pre><code>try:
      import requests
  except:
      import pip
      pip.main(['install', '-q', 'requests==2.0.1','PyYAML==3.11'])
      import requests
  </code></pre>
  
  <p>I have tried using importing main from pip._internal and using pipmain instead of pip.main() but it did not help.</p>
  
  <p>I am on <code>pip version 9.0.1</code> and <code>python 2.7</code></p>