This introduces some more conceptual changes to Shaman. The most important
one is that there is no longer a "checkout ID", but a "checkout path".
The Shaman client can request any subpath of the checkout directory,
so that it can handle things like project- or scene-specific prefixes.
This also requires that `TaskID` is no longer a custom type, because that
would cause import cycles. The alternative would be to put the generated
mocks directly into the `worker` package, but I didn't think that was
particularly nice. Maybe this'll be reconsidered later.
