Remove flake utils

[andreabedini]

Trying to tidy up the top level flake.

I have checked that 5f00c25 does not change any derivation iun hydraJobs (excluding the ones depending on the git rev).

Marking it as draft because this probably makes evaluation worse as it re-evaluates

self.internal.compat { inherit system; })

many times. I need to factor that out (likely merging/replacing it with legacyPackages).

[hamishmack]

It would be nice to have one less dependency, but it would be better if we had a solution that also worked for examples and code templates using haskell.nix. Until then I think perhaps we should at keep the flake-utils input (so as not to break existing code that follows it). Perhaps we could still avoid using flake-utils in flake.nix, if you think it is more readable.

[angerman]

We can at least go back to upstream for now numtide/flake-utils#97.

[hamishmack]

Now we can advise people to switch to inputs.flake-utils.url = "github:numtide/flake-utils". No need to keep the input for .follows support (unless there is a better option coming).

[andreabedini]

This ended up being quite of a work of the main nix entrypoints (flake.nix, default.nix, shell.nix and ci.nix).
Everything is mostly flake.nix centric now, and default.nix tries its best to maintain compatibility with the pre-flake default.nix.