Resolver: evaluate the five dependency kinds

[Maximilian-Nesslauer]

Part 4 of the split in #25, blocked on the Core model PR.

Teach ModDependencyResolver the semantics of ModDependencyKind per RFC #31 (content-manager-design):

• required installs always, recommends is selected by default and deselectable, suggests is listed but not selected, optional is not installed by default.
• conflict blocks installing both within the bounds; no bounds means every version conflicts.
• Version bounds are inclusive min/max with an open upper end when absent.
• Tests per kind, including the no-bounds conflict case.