Currently the selectors only have access to a list of labels. But if they could access the entire task definition, they would be able to accomplish a lot more. I had previously implemented this as part of bug 1499822, but this will be useful for other purposes as well, and I'm not sure when that bug will be able to land. So spinning this change into a new bug to get it landed sooner.

Currently selectors that generate the taskgraph receive a list of task labels back. While this works, it isn't very robust and limits the selectors in what they can accomplish. For example, it means we need to use regexes to map test suites to tasks. If we had the full taskgraph, we could use filter functions instead. There are also new selectors in the works which will need this (like ./mach try chooser). Finally the try syntax selector would also need this if we ever migrate it to use the 'try_task_config.json' format.

Pushed by ahalberstadt@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/306f3bd4760a [tryselect] Pass full taskgraph into selectors, r=marco