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 problem was exposed by unit-tests in loopback-workspace[1].

[1] https://github.com/strongloop/loopback-workspace