For the time being, I've pushed a change which uses an empty folder as the package source. I suppose the only thing missing really is proof that the mocked packages used for all other tests have correct data for the package they represent. 

One neat way to do that is have a test data generation tool which queries a real package and serializes the results to something like json, which we could then use in the tests much more freely than a binary format since they're quick to read/copy and diff nicely in VCS. Probably not worth the effort though given that a single manual run of the tool anywhere will pretty much verify this aspect.