ocamlPackages.buildDunePackage: support fixed point args with `lib.extendMkDerivation`

doc/languages-frameworks/ocaml.section.md

···
   ppx_let,
 }:
 
-buildDunePackage rec {
+buildDunePackage (finalAttrs: {
   pname = "angstrom";
   version = "0.15.0";
 
···
   src = fetchFromGitHub {
     owner = "inhabitedtype";
     repo = "angstrom";
-    tag = version;
+    tag = finalAttrs.version;
     hash = "sha256-MK8o+iPGANEhrrTc1Kz9LBilx2bDPQt7Pp5P2libucI=";
   };
 
-  checkInputs = [
-    alcotest
-    ppx_let
-  ];
   buildInputs = [ ocaml-syntax-shims ];
+
   propagatedBuildInputs = [
     bigstringaf
     result
   ];
+
   doCheck = lib.versionAtLeast ocaml.version "4.05";
+  checkInputs = [
+    alcotest
+    ppx_let
+  ];
 
   meta = {
     homepage = "https://github.com/inhabitedtype/angstrom";
···
     license = lib.licenses.bsd3;
     maintainers = with lib.maintainers; [ sternenseemann ];
   };
-}
+})
 ```
 
 Here is a second example, this time using a source archive generated with `dune-release`. It is a good idea to use this archive when it is available as it will usually contain substituted variables such as a `%%VERSION%%` field. This library does not depend on any other OCaml library and no tests are run after building it.
···
   buildDunePackage,
 }:
 
-buildDunePackage rec {
+buildDunePackage (finalAtts: {
   pname = "wtf8";
   version = "1.0.2";
 
   minimalOCamlVersion = "4.02";
 
   src = fetchurl {
-    url = "https://github.com/flowtype/ocaml-wtf8/releases/download/v${version}/wtf8-v${version}.tbz";
+    url = "https://github.com/flowtype/ocaml-wtf8/releases/download/v${finalAtts.version}/wtf8-v${finalAtts.version}.tbz";
     hash = "sha256-d5/3KUBAWRj8tntr4RkJ74KWW7wvn/B/m1nx0npnzyc=";
   };
 
···
     license = lib.licenses.mit;
     maintainers = [ lib.maintainers.eqyiel ];
   };
-}
+})
 ```
 
 The build will automatically fail if two distinct versions of the same library

pkgs/build-support/ocaml/dune.nix

···
   dune_3,
 }:
 
-{
-  pname,
-  version,
-  nativeBuildInputs ? [ ],
-  enableParallelBuilding ? true,
-  ...
-}@args:
+lib.extendMkDerivation {
+  constructDrv = stdenv.mkDerivation;
+  excludeDrvArgNames = [
+    "minimalOCamlVersion"
+    "duneVersion"
+  ];
+  extendDrvArgs =
+    finalAttrs:
+    {
+      pname,
+      version,
+      nativeBuildInputs ? [ ],
+      enableParallelBuilding ? true,
+      ...
+    }@args:
 
-let
-  Dune =
     let
-      dune-version = args.duneVersion or "3";
+      Dune =
+        let
+          dune-version = args.duneVersion or "3";
+        in
+        {
+          "1" = dune_1;
+          "2" = dune_2;
+          "3" = dune_3;
+        }
+        ."${dune-version}";
     in
-    {
-      "1" = dune_1;
-      "2" = dune_2;
-      "3" = dune_3;
-    }
-    ."${dune-version}";
-  stdenv' = args.stdenv or stdenv;
-in
 
-if args ? minimalOCamlVersion && lib.versionOlder ocaml.version args.minimalOCamlVersion then
-  throw "${pname}-${version} is not available for OCaml ${ocaml.version}"
-else
+    if args ? minimalOCamlVersion && lib.versionOlder ocaml.version args.minimalOCamlVersion then
+      throw "${pname}-${version} is not available for OCaml ${ocaml.version}"
+    else
+      {
+        name = "ocaml${ocaml.version}-${pname}-${version}";
 
-  stdenv'.mkDerivation (
-    {
+        strictDeps = true;
 
-      inherit enableParallelBuilding;
-      dontAddStaticConfigureFlags = true;
-      configurePlatforms = [ ];
+        inherit enableParallelBuilding;
+        dontAddStaticConfigureFlags = true;
+        configurePlatforms = [ ];
 
-      buildPhase = ''
-        runHook preBuild
-        dune build -p ${pname} ''${enableParallelBuilding:+-j $NIX_BUILD_CORES}
-        runHook postBuild
-      '';
-      checkPhase = ''
-        runHook preCheck
-        dune runtest -p ${pname} ''${enableParallelBuilding:+-j $NIX_BUILD_CORES}
-        runHook postCheck
-      '';
-      installPhase = ''
-        runHook preInstall
-        dune install --prefix $out --libdir $OCAMLFIND_DESTDIR ${pname} \
-         ${
-           if lib.versionAtLeast Dune.version "2.9" then
-             "--docdir $out/share/doc --mandir $out/share/man"
-           else
-             ""
-         }
-        runHook postInstall
-      '';
+        nativeBuildInputs = [
+          ocaml
+          Dune
+          findlib
+        ]
+        ++ nativeBuildInputs;
 
-      strictDeps = true;
+        buildPhase =
+          args.buildPhase or ''
+            runHook preBuild
+            dune build -p ${pname} ''${enableParallelBuilding:+-j $NIX_BUILD_CORES}
+            runHook postBuild
+          '';
 
-    }
-    // (builtins.removeAttrs args [
-      "minimalOCamlVersion"
-      "duneVersion"
-    ])
-    // {
+        installPhase =
+          args.installPhase or ''
+            runHook preInstall
+            dune install --prefix $out --libdir $OCAMLFIND_DESTDIR ${pname} \
+             ${
+               if lib.versionAtLeast Dune.version "2.9" then
+                 "--docdir $out/share/doc --mandir $out/share/man"
+               else
+                 ""
+             }
+            runHook postInstall
+          '';
 
-      name = "ocaml${ocaml.version}-${pname}-${version}";
+        checkPhase =
+          args.checkPhase or ''
+            runHook preCheck
+            dune runtest -p ${pname} ''${enableParallelBuilding:+-j $NIX_BUILD_CORES}
+            runHook postCheck
+          '';
 
-      nativeBuildInputs = [
-        ocaml
-        Dune
-        findlib
-      ]
-      ++ nativeBuildInputs;
-
-      meta = (args.meta or { }) // {
-        platforms = args.meta.platforms or ocaml.meta.platforms;
+        meta = (args.meta or { }) // {
+          platforms = args.meta.platforms or ocaml.meta.platforms;
+        };
       };
-
-    }
-  )
+}