Fix launch errors, improve Pods API, use hook abstraction

Sorry for the largeness of this PR, just took a while to get it all together. @mpuncel @petertseng 

This extracts the preparer's orchestration logic to a package that can be tested. It does not solve the self-hosting issue yet, but gets us most of the way there.
This creates a "current" directory for that specific hoist launchable. Since we don't actually refer to anything "current" in the runit services, I'm not sure this is a good idea. @mpuncel lmk what you think.