title: npm package is not listed in project's package.json but no extraneous error
body: <p>I see a weird situation in my project.
  There are 3 packages not listed in <code>package.json</code>, but installed during the development process. </p>
  
  <p>In my understanding, <code>npm list</code> should show me <code>extraneous error</code>. However, no error displayed. I am wondering how npm decides which package is extraneous or not?</p>
  
  <p>The three packages are <code>async</code>, <code>debug</code> and <code>mime</code>. and I am using the <code>npm 1.4.28</code></p>