Support loading of mixins

See https://github.com/strongloop/loopback-datasource-juggler/pull/201
My understanding of https://github.com/strongloop/loopback-datasource-juggler/pull/229 is that a mixing can be an object too?

Please add debug statements to make troubleshooting easier, especially when the condition on L148 is false. See `defineModels` for inspiration.