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 feed behavior.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19280