Nix: Expose package set separately

Exposes the OCaml packages through the Nix flake. This can be used to access the same versions of OCaml-related packages from other repositories, for instance.

Try it out on Mac:

nix build --no-link --print-out-paths gitlab:tezos/tezos/ole/nix-package-set#ocamlDependencies.aarch64-darwin.ocaml

Or on Linux:

nix build --no-link --print-out-paths gitlab:tezos/tezos/ole/nix-package-set#ocamlDependencies.x86_64-linux.ocaml