Include loopback in model sources

As of loopback 2.5, built-in models are defined in a layout compatible
with loopback-boot and loopback-workspace.

Adding loopback to model sources makes it is easier to programatically
detect which built-in models are used by the facet.
Merge pull request #163 from strongloop/feature/reference-builtin-models

Include loopback in model sources