Support require one or the other nature

Currently one can only require another nature by id, or one can only allow one or the other nature.
What I recently came along (and seem not supported), is that one wants to require any of a set of natures (e.g. nature `a` or nature `b`) because both are exclusive (you can only have `a` or `b` on the project) but my nature wants at least one of them.

So from an extension point of view I then need to mark one as the "preferred" that is added if none of the ones I want are available.

まず、拡張ポイントと、idでnatureを要求し、exclusive naturesを許可する既存の処理を見つけます。natureの要件がどのように評価されるかを追跡し、次に、集合から任意のnatureを要求する場合に完了とみなす条件と、natureが存在しない場合にpreferred natureを追加する条件を定義します。

索引モデルが issue の本文から書いたものです。