title: Check if package is installed using pipenv
body: <p>Is there a way to check if a particular package is installed using pipenv?
  I have a <code>Makefile</code> which runs <code>pipenv run &lt;command&gt;</code> but I want to show a friendly message if the package is not installed and ask users to run <code>pipenv install</code>. </p>
  
  <p>Currently the run command just creates new <code>vitrualenv</code> and just fails with command not found.</p>