Retire staged Yarn dependencies during Nix installation (#473)

test/yarn-dependencies/yarn-project.nix

@@ -0,0 +1,31 @@
+# Tiny generated-project interface fixture. Real offline Yarn installation
+# remains covered by the consuming Studio and Package-builder checks.
+{ stdenvNoCC, nodejs }:
+{ src, overrideAttrs ? null }:
+let
+  project = stdenvNoCC.mkDerivation {
+    name = "yarn-dependencies-transfer-fixture";
+    inherit src;
+    configurePhase = ''
+      mkdir -p node_modules/fixture node_modules/.bin .yarn/cache
+      printf '%s\n' '{"main":"index.cjs"}' > node_modules/fixture/package.json
+      printf '%s\n' 'module.exports = {value: "retained dependency"};' > node_modules/fixture/index.cjs
+      ln node_modules/fixture/index.cjs node_modules/fixture/twin.cjs
+      ln -s fixture node_modules/alias
+      printf '%s\n' '#!${nodejs}/bin/node' 'console.log("linked executable");' > node_modules/fixture/cli.cjs
+      chmod +x node_modules/fixture/cli.cjs
+      ln -s ../fixture/cli.cjs node_modules/.bin/fixture
+      printf '%s\n' 'hidden dependency data' > node_modules/.fixture-data
+      printf '%s\n' 'disposable linker state' > node_modules/.yarn-state.yml
+      printf '%s\n' 'disposable archive' > .yarn/cache/fixture.zip
+    '';
+    postInstall = ''
+      test ! -e node_modules
+      test ! -e .yarn/cache
+      test ! -e "$out/.yarn-state.yml"
+      test ! -e "$out/node_modules"
+      test "$(stat -c %i "$out/fixture/index.cjs")" = "$(stat -c %i "$out/fixture/twin.cjs")"
+    '';
+  };
+in
+if overrideAttrs == null then project else project.overrideAttrs overrideAttrs