R1:346ffc51e100

Modularize "HarbormasterBuildableTransaction"

Summary: Ref T13072. Trivially convert this into a modular transaction type.

Test Plan: Issued commands to a buildable.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21694