Add kind that demonstrates how to modify the upstream graph in a transform

[bhearsum]

This is a very rough prototype for what's been discussed in #417 and taskcluster/taskgraph#424. In this example, the decision about which tasks merge-corpus should depend on are being deferred and handled in the new modify_graph transform, which runs as part of the modify-graph task that happens right before all.

Because we're handling the dependencies later, we must also handle the fetches later - which means that knowledge about those artifact names moves out of the merge-corpus kind and into this transform.

I haven't tested this besides some taskgraph full -Y inspection, but it looks plausible - and the code certainly runs at the appropriate time, and has access to the necessary data to make the changes.

Any tasks that you need to modify or access must be in a kind listed in the modify-graph kind's kind-dependencies, otherwise they will not show up in config.kind_dependencies.tasks.

And at the risk of being annoying and repeating myself, I will offer a couple of cautions:

• This sort of thing goes against the design of taskgraph. It could break without warning, and may not fit in well with future taskgraph features
• We used to manage Firefox build & release tasks/graphs with similar imperative code. Taskgraph was, in part, a reaction to how unmaintainable and spaghetti-like that became.