upgrade: new option: --latest (-l)
Select latest version of package (if it is not pinned), and print error if it cannot be installed due to other dependencies. Together with --available, it selects the latest package which is present at least in some repository. This also fixes few solver issues with ordering of package selection that got quite apparent with this flag. Namely, we cannot "lock" a package until it's reverse dependencies are locked or not all of the solver flags are propagated properly.
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