Compile only unique Docker stages determined from artifacts

Summary:
The previous implementation for iterating over defined artifacts did not
take into account the possibility for multiple artifacts with the same
from variant name and would compile stages with duplicate names. This
fixes that behavior by iterating over a unique set of names.

Test Plan: Run go test ./... or arc unit.

Reviewers: thcipriani, mmodell, Release-Engineering-Team

Reviewed By: thcipriani, Release-Engineering-Team

Tags: Release-Engineering-Team

Differential Revision: https://phabricator.wikimedia.org/D758