Problem: We are using a homegrown hack to detect cycles

[clacke]

Solution: Use graph instead.

https://docs.racket-lang.org/graph/index.html#%28def._%28%28lib._graph%2Fmain..rkt%29._scc%29%29

racket2nix itself has no cycles in its transitive dependencies, so it's fine to bring on a dependency.

Solve stchang/graph#35 first.