Rename "HarbormasterBuild" methods to prepare for use of the "BuildMessages" table

Summary:
Ref T13072. Rename various "command" properties to "message" properties, to prepare for merging "HarbormasterCommand" into "HarbormasterMessage".

This change only renames variables and methods and should not affect program behavior.

Test Plan: Grepped for affected symbols, found no unmodified hits.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21683