Refactoring module dependencies

While working on #20218 (closed) I noticed that the code handling module dependencies seemed inefficient because it used sorted lists instead of sets. CI seems to agree.

      MultiLayerModules(normal) ghc/alloc  4130851520.0  2981473072.0 -27.8%
                 T13719(normal) ghc/alloc  4313296052.0  4151647512.0  -3.7%