Motivation for this change

The initial motivation was to fix parmap with OCaml 4.09. At version 1.1, parmap depends on graphics which is shipped with OCaml < 4.09; when distributed as a separate library, it is built by dune 2 and seems to impose to dependent libraries to use dune 2 as well. This is why I wanted an easy way to select the version of dune.

FTR, here is a possible way to fix parmap after this PR:

--- a/pkgs/development/ocaml-modules/parmap/default.nix
+++ b/pkgs/development/ocaml-modules/parmap/default.nix
@@ -1,4 +1,4 @@
-{ lib, buildDunePackage, fetchzip }:
+{ lib, buildDunePackage, fetchzip, dune-configurator, graphics }:
 
 buildDunePackage rec {
   pname = "parmap";
@@ -9,6 +9,10 @@ buildDunePackage rec {
     sha256 = "13ahqaga1palf0s0dll512cl7k43sllmwvw6r03y70kfmky1j114";
   };
 
+  useDune2 = graphics != null;
+
+  buildInputs = [ graphics ] ++ lib.optional useDune2 dune-configurator;
+

Updating parmap (as done in #82585) is a much simpler way to solve this particular issue.

Nonetheless, I think that getting rid of buildDune2Package is a good idea.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.