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.

Requires https://github.com/strongloop/loopback-boot/pull/60

A follow-up for https://github.com/strongloop/loopback/pull/635, a part of https://github.com/strongloop-internal/scrum-loopback/issues/34.

/to @ritch please review