This addresses pulumi/pulumi#446: what we used to call "package" is
now called "project". This has gotten more confusing over time, now
that we're doing real package management.
Also fixespulumi/pulumi#426, while in here.
Instead of having information stored in the checkpoint file, save it
in the Pulumi.yaml file. We introduce a new section `stacks` which
holds information specific to a stack.
Next, we'll support adding configuration information that applies
to *all* stacks for a Program and allow the stack specific config to
overwrite or augment it.