Stage jars should not have to be jar-with-dependencies

[laserval]

Quoting @jwestberg :

I've been thinking a lot about the stage api issue, and with the current way we run stages, it would not be a major overhaul to allow non-runnable jars where the core provides its api.

Stages currently have to be runnable jars containing all their dependencies. This leads to both bloat (dependencies are duplicated) and to problems where the stage API might not be in sync with the Core API.