Support loading of mixins

See https://github.com/strongloop/loopback-datasource-juggler/pull/201
As I have commented in [https://github.com/strongloop/loopback-datasource-juggler/pull/229], the mixin name should be normalized here, not in the juggler.

At least these two naming conventions should be supported: 'international-address.js` and`InternationalAddress.js`.