autopkgtest artifacts should have relationships

Contrary to what the documentation 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: