This plumbs through support for building multi-arch SPDX SBOMs largely based on Puerco's outline, but with a few
adaptations. I added a few minor refactorings to try to enable consistency across the Image/Index SBOMs.
Related: https://github.com/google/ko/issues/655
* Unconditionally set the base image annotation.
Previously we only set this annotation when our base image was a tag, but this means when folks actually follow the best practice of use digest base images they get strictly worse resulting images!
It sounds like maybe the original motivation for this condition was that it was supposed to contain the mutable reference (tag), but I don't see anything detailing such a restriction (just an example), so there should be nothing
precluding this from the spec.
* Fold assignment into map initialization.
Matt Moore