Depends on D19278. Ref T13110. This moves most of the structural logic for publishing builds to BuildableEngine and provides a bin/harbormaster publish to make publishing easy to retry/debug.

This intentionally removes the bit which actually does anything when builds publish. Followup changes will implement application-specific versions of the publishing logic in Differential and Diffusion.

Ran bin/harbormaster publish Bxxx, saw it do nothing (but not crash).