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
