Require data-sources directory in addition to models directory in `app.boot()`.

This prevents a potential issue where a model is loaded from config JSON that specifies use a datasource
that is loaded via JS (e.g. via a module in `./data-sources`). The model would typically be loaded before the datasource in this case, leading to an assertion error as the datasource is not properly set up.

This allows the files in `data-sources` to be loaded with the full `appConfig` attached to the app object, allowing a datasource module to use environment configuration in its configuration.