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.
Previously, when uploading a projectm to the service, we would only
upload the folder rooted by the Pulumi.yaml for that project. This
worked well, but it meant that customers needed to structure their
code in a way such that Pulumi.yaml was always as the root of their
project, and if they wanted to share common files between two projects
there was no good solution for doing this.
This change introduces an optional piece of metadata, named context,
that can be added to Pulumi.yaml, which allows controlling the root
folder used for computing the root folder to archive from. When it is
set, it is combined with the location of the Pulumi.yaml file for the
project we are uploading and that folder is uses as the root of what
we upload to the service.
Fixes: #574
