def prefetch(reqs)
        names = reqs.map { |r| r.dependency.name }
        needed = names.find_all { |d| !@data.key?(d) }

        return if needed.empty?

        u = URI.parse "http://rubygems.org/api/v1/dependencies?gems=#{needed.join ','}"
        str = Net::HTTP.get(u)

        Marshal.load(str).each do |ver|
          @data[ver[:name]] << ver
        end
      end
A hint run by the resolver to allow the Set to fetch
data for DependencyRequests +reqs+.