R1:c42bbd6f5cff

Rename HarbormasterBuildMessage "buildTargetPHID" to "receiverPHID"

Summary: Ref T13054. Companion storage change for D19062.

Test Plan: Applied migration and adjustments. Viewed messages in Harbormaster; created them with `harbormaster.sendmessage`; processed them with `bin/phd debug task`.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13054

Differential Revision: https://secure.phabricator.com/D19063