Deal with duplicates in the solution space of a dep

While we process the possible solutions we might modify other solutions
like discarding their candidates and such, so that then we reach them
they might no longer be proper candidates. We also try to drop
duplicates early on to avoid the simple cases of these which
test-explore-or-groups-in-markinstall triggers via its explicit
duplication but could also come via multiple provides.

It only worked previously as were ignoring current versions which
usually is okay expect if they are marked for removal and we want to
reinstate them so the ProblemResolver can decide which one later on.

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