MikeLing initially did this in bug 1287018. The intent of this conditional was
to make optimization faster by not even checking most tasks, based on the
assumption that if the prerequisite to a task has changed (for example, a
docker image or a build), then naturally we will want to execute that task.
However, as we have developed actual optimization methods, this has proven not
to be the case: we might want to optimize a test out if its inputs have not
changed, even if a new installer has been built. Similarly, SETA may optimize
tasks out even if their inputs have changed.
MozReview-Commit-ID: LgHET3Z84GB
* Implement & document optimization (although legacy kind doesn't do much of it)
* Introduce `optimize_target_tasks` parameter to control whether tasks in the
target set can be optimized (no for try, yes for most other branches)
* Refactor to include resolved taskIds in the optimized task graph
* Include a `label-to-taskid.json` artifact.
* Introduce {'task-reference': '... <dependency-name> ...'} for referring to
parent tasks' taskId.
MozReview-Commit-ID: LWvlWNz49U5
