Support imports property in swagger export

[rlmartin]

If service.imports is non-empty, we should recursively pull down the referenced service.json and add it to the swagger.json. We'll need to figure out the logic around how to choose which definition should take precedence when there are collisions. For example: what if the imported definition has the same model but a different definition of the model? Should the models be combined (like a union)? Should the imported one replace the original one? Should the original one remain unaltered? Same thing goes for all levels of the definition, i.e. collisions on resources, parameters, operations, etc.

[rlmartin]

The hbcmobile-wishlist-api.json test resource uses imports, and the invalid swagger spec that is produced should become valid once this issue is resolved.