generate an error of trust section list packages not used

[rn]

While adding trust entries for packages as part of linuxkit/linuxkit#1909 I noticed that some YAML files had entries which did not match any images listed in the image.

I think we should make the build error out in this case as it forces tighter, more correct YAML files

@riyazdf WDYT?

[riyazdf]

👍 Agreed. This can live in the general config validation code as we flesh it out.

I want to bubble up some of the trust code that does full name resolution (ex: so that we can understand orgs like library) first, which will be a prerequisite for this.