Improve Scope Resolution

`PolicyFinder` fails to find a `Scope` class when used with inheritance. For example:

    class ContentPolicy
      ...

      class Scope
        def resolve
          ...
        end
      end
    end

    Pundit::PolicyFinder.new(Content).scope
    => ContentPolicy::Scope

    class PhotoPolicy < ContentPolicy
      class Scope
        def resolve
          ...
        end
      end
    end

    Pundit::PolicyFinder.new(Photo).scope
    => PhotoPolicy::Scope

    class ArticlePolicy < ContentPolicy
    end

    Pundit::PolicyFinder.new(Article).scope
    => nil

It looks like this happens due to the behaviour of `#constantize` and `#safe_constantize`:

    "ContentPolicy::Scope".safe_constantize
    => ContentPolicy::Scope
    "ArticlePolicy::Scope".safe_constantize
    => nil

With this change, `PolicyFinder` stops trying to "find" scopes. The policy is found in the normal way (using `safe_constantize` rather than `constantize`) and interrogated for the `Scope` class using `#const_get`.