buildDartApplication: Move package sources to nativeBuildInputs

They are only needed at build time, after all.

pkgs/build-support/dart/build-dart-application/default.nix

···
       jq
     ] ++ lib.optionals stdenv.isDarwin [
       darwin.sigtool
-    ];
-
-    buildInputs =
+    ] ++
       # Ensure that we inherit the propagated build inputs from the dependencies.
       builtins.attrValues pubspecLockData.dependencySources;