title: Lerna fails with 404 when trying to link dependency
body: <p>My structure is as follows:</p>
  
  <ul>
  <li>package.json</li>
  <li>lerna.json</li>
  <li>packages
  
  <ul>
  <li>myproj-util
  
  <ul>
  <li>package.json</li>
  </ul></li>
  <li>myproj-schema
  
  <ul>
  <li>package.json -- has dev-depenency on <code>myproj-util</code></li>
  </ul></li>
  </ul></li>
  </ul>
  
  <p>Neither project is published to npm yet. </p>
  
  <p>When I run <code>lerna boostrap</code> I get:</p>
  
  <pre><code>❯ lerna bootstrap
  lerna info version 2.4.0
  lerna info versioning independent
  lerna info Bootstrapping 2 packages
  lerna info lifecycle preinstall
  lerna info Installing external dependencies
  lerna ERR! execute callback with error
  lerna ERR! Error: Command failed: npm install
  lerna ERR! npm ERR! code E404
  lerna ERR! npm ERR! 404 Not Found: myproj-util@*
  </code></pre>
  
  <p>My understanding <a href="https://github.com/lerna/lerna#how-bootstrap-works" rel="noreferrer">from the docs</a> is that when running <code>lerna bootstrap</code> it should "check if each dependency is part of the Lerna repo" so it should just be symlinked.</p>
  
  <p>So... why is it not just symlinking my internal dependency? The dependency is under <code>packages/</code> and the folder name and package name match exactly.</p>