* 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
The `taskgraph` package generates TaskCluster task graphs based on collections
of task "kinds". Initially, there is only one kind, the "legacy" kind, which
reads the YAML files from `testing/taskcluster/tasks` to generate the task
graph.
Try syntax is implemented by filtering the tasks in the taskgraph after it has
been created, then extending the result to include any prerequisite tasks.
A collection of `mach taskgraph` subcommands are provided for developers to
extend or debug the task-graph generation process.
MozReview-Commit-ID: 1TJCns4XxZ8
