USN notification service is failing to report affected binary packages if primed-stage-packages is present but empty in manifest.yaml and dpkg.list was used to fake stage packages

[Emilia Torino]

When the support for reading snaps stage-packages from the primed-stage-packages manifest key was added, it was considered that if that the primed-stage-packages key was present, the packages inside this key should take precedence over all stage-packages of each part.

This is correct in general scenarios, but is failing under special scenarios such as the core snap, where the primed-stage-packages key is present in the snap manifest but is always empty.

$ review-tools.check-notices core18_2066.snap
{
  "core18": {
    "2066": {}
  }
}

With 2021/05/27 USN DB it should report:
{
  "isc-dhcp-client": [
    "4969-1"
  ],
  "liblz4-1": [
    "4968-1"
  ]
}

There is a related bug which is related to properly parsing binary names from a dpkg.list file which also affects this result: https://bugs.launchpad.net/review-tools/+bug/1930105. Both bugs need to be resolved together for this result to be properly provided.