Previously, you could pass an explicit path to a Pulumi program when
running preview or update and the tool would use that program when
planning or deploying, but continue to write state in the cwd. While
being able to operate on a specific package without having to cd'd all
over over the place is nice, this specific implemntation was a little
scary because it made it easier to run two different programs with the
same local state (e.g config and checkpoints) which would lead to
surprising results.
Let's move to a model that some tools have where you can pass a
working directory and the tool chdir's to that directory before
running. This way any local state that is stored will be stored
relative to the package we are operating on instead of whatever the
current working directory is.
Fixes#398
065f6f2b42