dart.list-dart-deps: Add list-dart-deps function

This uses `dart pub deps` (https://dart.dev/tools/pub/cmd/pub-deps) to retrieve information about project dependencies.

It requires a fetch-dart-deps derivation as input.

pkgs/build-support/dart/list-dart-deps/default.nix

@@ -0,0 +1,28 @@
+{ stdenv
+, dart
+, jq
+}:
+deps:
+
+builtins.fromJSON (builtins.readFile (stdenv.mkDerivation {
+  name = "${deps.name}-list.json";
+  nativeBuildInputs = [ deps dart jq ];
+
+  unpackPhase = ''
+    runHook preUnpack
+    ln -s "${deps.files}"/pubspec/* .
+    runHook postUnpack
+  '';
+
+  configurePhase = ''
+    runHook preConfigure
+    dart pub get --offline
+    runHook postConfigure
+  '';
+
+  buildPhase = ''
+    runHook preBuild
+    dart pub deps --json | jq .packages > $out
+    runHook postBuild
+  '';
+}))