Fix serializer for erratum pkglists

[goosemania]

It didn't affect publish results, however it did affect applicability
calculations and erratum content which user queries via API.

Kudos to Hyu (reporter of the issue) for the patch.

closes #4880
https://pulp.plan.io/issues/4880

[goosemania]

Verification steps (2 repos with the same errata id but different pkglists are needed):

# create and sync one repo
$ pulp-admin rpm repo create --repo-id base_repo --relative-url base_repo --download-policy on_demand --feed $URL_TO_BASE_REPO
$ pulp-admin rpm repo sync run --repo-id base_repo

# create and sync other repo (which has at least one erratum with the same id as in base_repo but different pkglist)
$ pulp-admin rpm repo create --repo-id debuginfo_repo --relative-url debuginfo_repo --download-policy on_demand --feed $URL_TO_DEBUGINFO_REPO
$ pulp-admin rpm repo sync run --repo-id debuginfo_repo

# search for the erratum of interest, check that both pkglists are present and not only one like before
$ pulp-admin -vvv rpm repo content errata --repo-id base_repo --str-eq 'id=<errata-id-which-is-present-in-both-repos>'