and use it to report module loops nicely

This fixes Trac #5307. Now we get

    Module imports form a cycle:
             module `M8' (.\M8.hs)
            imports `M1' (M1.hs)
      which imports `M9' (.\M9.hs-boot)
      which imports `M8' (.\M8.hs)

And the algorithm is linear time.