Fix loading of loopback dependencies.

Loopback-explorer loads express and strong-remoting from loopback's `node_modules` folder. This fails when one of those dependencies was installed to the parent project, e.g. as a peer dependency.

    app/node_modules
     +- express # <- express installed as peer to loopback
     +- loopback
         +- node_modules # <- express is not there

As of this commit, loopback-explorer will retry the `require()` call without `loopback/node_modules` prefix if the first require fails.

    1. `require('loopback/node_modules/express')`
    2. `require('express')`

The change should fix (some of) unit-tests failures in [loopback-workspace](https://github.com/strongloop/loopback-workspace)