Make output order consistent

[Fuuzetsu]

I just adjusted a couple of dependencies in a project and it resulted in 2KLOC change in packages.nix (pruned). I suspect change in transitive dependencies caused some packages to be resolved sooner/later and therefore be placed in different place. If packages are sorted by name, diffs should be much smaller.

I don't have a public test-case, sorry.

[4e6]

We are using Data.Map, Data.Set, and Data.Graph to store and manipulate loaded packages. They all require Ord class on construction, thus should have a stable order. I also can not think of a place where reordering can happen.

The most obvious thing would be to sort resulting packages set as a final step.

[4e6]

The order of extra deps in stack.yaml should matter. It may affect the reachableDependencies DFS algorithm. It is most likely the source of a problem.