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.
I think we should refer to current in runit services. I think people are always going to want to be able to flip a current symlink and sv restart if (or in case) things get hairy.