destutil: if wdp is obsolete, update to the closest non-obsolete ancestor

As the original comments suggest, using prune as a model here was an existing
idea, and now this patch implements it.

I think it would be even better to do what solveobswdp() from evolve does (in
short, it allows to update to a successor of the closest ancestor even if that
ancestor is obsolete), but that is outside of this series' scope.

Differential Revision:
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