autopkgtest artifacts should have relationships
Contrary to what the [documentation](https://freexian-team.pages.debian.net/debusine/reference/artifacts/specs/autopkgtest.html) states, the autopkgtest artifacts have no relationships currently.
Currently the documentation states this:
> Relationships:
> * relates-to: the artifacts used as input that are part of the source package being tested. They can be of types debian:source-package, debian:upload, debian:binary-packages or debian:upload
When we get to add them, it probably make senses to not follow the current documentation and standardize the relationships to link only the `debian:source-package` and all the `debian:binary-package` instead (i.e. we should expand the `debian:upload` artifact and link to the underlying artifacts). And maybe use a "built-using" relationship since that makes more sense than "relates-to" ? (Not sure of that last suggestion)
Example autopkgtest: https://debusine.debian.net/debian/developers/artifact/1366788/
Notice the lack of "Relations" section:

issue