This adds artifact dependencies and parallisation to Harbormaster. Each build step now has an output artifact called "Build State", where the key is the PHID of the build step. Build steps have a depends_on field which indicates what other build states they depend on.
In the plan editor, it looks like this:
To keep things simple and to ensure you can't create circular dependencies without knowing about it, the user still needs to order their build steps such that a build step only refers to artifacts before it. Otherwise you get the 'Artifact not available at this time during the build' message that you currently get.
The Depends On field does not display for the first step in the build.
NOTE: This does not yet consider non-build state dependencies. i.e. if you have a "Publish Fragment" step that relies on a file artifact, it won't be included in the dependency calculations.