Join images always generate underlying packages

Line 828 in 5c84e5b

wantsArtifact := true

This should happen always for the package that we want to build, but not for all the dependencies of each package in the list, we just want the artifact of the target one. We ideally would like to tie that behavior to --only-target-package.

The problem here is compile doesn't distinguish if we want to generate the artifact of the target, or of all the dependencies. In this case we should be able to distinguish, while in the copy case we can tie it to -only-target-package without issues, here we want to be able to treat both cases separately.

Also, as a reminder, the whole is tied to the metadata generation: we shouldn't assume a default behavior, but rely on whether --only-target-package is passed or not.