title: Require python imports to be modules
body: <p>The Google Style Guide for python states that one should:
  "Use imports for packages and modules only."</p>
  
  <p><a href="https://google.github.io/styleguide/pyguide.html#Imports" rel="noreferrer">https://google.github.io/styleguide/pyguide.html#Imports</a></p>
  
  <p>Is there a tool that flags violations of this suggestion?</p>
  
  <p>Pylint does NOT do it.  For example, following:
  <a href="https://stackoverflow.com/questions/29597618/is-there-a-tool-to-lint-python-based-on-the-google-style-guide">Is there a tool to lint Python based on the Google style guide?</a></p>
  
  <p>Creating a <code>test.py</code> the violates the guideline (<code>exists</code> is a function, not a module):</p>
  
  <pre><code>"""Test file for pylint"""
  from os.path import exists
  
  exists('/home')
  </code></pre>
  
  <p>Then, running pylint with the rc file does just fine:</p>
  
  <pre><code>$ pylint --rcfile=googlecl-pylint.rc -r n -s n  test.py
  $ echo $?
  0
  </code></pre>
  
  <p>Searching through the possible codes: <a href="http://pylint-messages.wikidot.com/all-codes" rel="noreferrer">http://pylint-messages.wikidot.com/all-codes</a>, I don't see anything that looks like it would warn against this.</p>
  
  <p>I have also not seen anything in pep8 or pyflakes that will catch this.</p>