Fixed package serialization so it displays content checksums.

[ipanova]

closes #8002
https://pulp.plan.io/issues/8002

[notest]
[nocoverage]

[pulpbot]

Attached issue: https://pulp.plan.io/issues/8002

[dralley]

There's probably an argument for making SingleArtifactContentSerializer [0] just inherit this as a mixin so it would be automatically included without needing to add it for each content type and each plugin. I'd be OK with it either way.

[0] https://github.com/pulp/pulpcore/blob/master/pulpcore/app/serializers/content.py#L26

As mentioned in IRC, I kinda wonder if we should be putting these checksums under a different namespace to avoid conflicts with fields on the content models, e.g. pulp_ prefix. That is probably a separate discussion and a separate issue. Doing so would eliminate the downside of including it automatically, though.

[ipanova]

@dralley this is indeed something worth discussing. Not every plugin would benefit from having checksums automatically included in the SingleArtifactContentSerializer. For example pulp_container plugin mostly cares about sha256 only and in case ContenChecksums serializer wiould be incorporated in to the SingleArtifactContentSerializer we would need to exclude all checksums form serialization except sha256.