R1:ada0c9126cf0

Provide a modular buildable transaction in Diffusion

Summary:
Depends on D19279. Ref T13110. This implements the existing publishing logic for buildables, but does so via ModularTransactions instead of a core transaction type.

Since each application is implementing build transactions independently, this removes the core type.

Next, Differential will get a similar treatment.

Test Plan: Used `bin/harbormaster publish` (with some commenting-out-guard-clauses) to publish a commit Buildable; saw unchanged…