This change just flows the project's "main" directory all the way
through to the plugins, fixing #667. In that work item, we discussed
alternative approaches, such as rewriting the asset paths, but this
is tricky because it's very tough to do without those absolute paths
somehow ending up in the checkpoint files. Just launching the
processes with the right pwd is far easier and safer, and it turns
out that, conveniently, we set up the plugin context in exactly the
same place that we read the project information.
b6a386995a