In the edge case when an updated package has different (or no)
alternatives groups, make sure to prune those that are in pkgdb
but not in the newly installed package.
A potentially common case of this is when a package that formerly
had alternatives gets removed and a transitional metapackage
takes its place (which has no alternatives).
When the new package has no dependencies, oldest next possible
alternatives group will be used. This is because that indicates
a removed package. When there are dependencies, the newest one
will be used; as this indicates a transitional package.
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