[Merge] ~juliank/ubuntu-release-upgrader:improve-obsoletes into ubuntu-release-upgrader:ubuntu/main

[Julian Andres Klode]

I think we likely want to ship this _after_ the current fixes in ubuntu/main have landed in -updates, as this is a bigger change for performance, but ymmv.
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https://code.launchpad.net/~juliank/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/465148
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