R1:72337dedaff9

Make Harbormaster input and output artifacts more explicit

Summary:
Ref T1049. In Harbormaster, build steps may have various inputs (like a host they should run on) and outputs (like a reference to an uploaded file).

- Currently, inputs aren't defined anywhere (except implicitly at runtime).
- Instead, define inputs explicitly.
- Currently, outputs are defined in a way that loses information when misconfigured (the keys will collide).
- Instead, define inputs and outputs so they work whether…