[Merge] lp:~rbalint/update-manager/fix-stale-pkg-refs into lp:update-manager

[Balint Reczey]

I considered replacing pkg with a proxy object, but the code would have been more confusing imo.
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https://code.launchpad.net/~rbalint/update-manager/fix-stale-pkg-refs/+merge/347195
Your team Ubuntu Core Development Team is requested to review the proposed merge of lp:~rbalint/update-manager/fix-stale-pkg-refs into lp:update-manager.