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

[Balint Reczey]

Julian provided and alternative fix that updates referenced pkg objects upon cache.open() which is fairly fast and would fix the issue in a different way.

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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.