Align with tools.build (future)

[seancorfield]

This is a follow-on from #82 to align the various tasks with how tools.build works and to clean up the exit logic on various tasks (now that -X handles that properly).

[seancorfield]

So far I've created hf.depstar.api/jar and hf.depstar.api/uber which are intended to be drop-in replacements for tools.build.api/jar and tools.build.api/uber but have depstar's logic for merging licenses etc.

The specific behavior of compile-clj and write-pom look a little harder to replace directly as drop-in functions, due to the assumptions baked into both of those about inputs and outputs so I'm inclined to not try to replicate those.

It would definitely make sense to have "complete" versions of these functions in the API tho', or at least align what's in hf.depstar to work better with -X and therefore be more easily callable from build.clj.