R1:c20b4e365b76

Move structural build publishing logic to BuildEngine, provide "bin/harbormaster publish"

Summary:
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.

Test Plan…