fixup-yarn-lock: split out from prefetch-yarn-deps, cleanup installing

To reduce dependencies (mainly nix-prefetch-git and through that git,
git-lfs) when we just need to fixup a lock file, eg when building electron.
This also tries to avoid needless rebuilds when eg. golang is updated.

Also this cleans up and combined the build/installPhase of both tools to
be a lot simpler.

pkgs/build-support/node/fetch-yarn-deps/default.nix

···
     name = "prefetch-yarn-deps";
 
     dontUnpack = true;
+    dontBuild = true;
 
     nativeBuildInputs = [ makeWrapper ];
-    buildInputs = [ coreutils nix-prefetch-git nodejs-slim nix ];
+    buildInputs = [ nodejs-slim ];
+
+    installPhase = ''
+      runHook preInstall
 
-    buildPhase = ''
-      runHook preBuild
+      mkdir -p $out/bin $out/libexec
 
-      mkdir libexec
       tar --strip-components=1 -xf ${yarnpkg-lockfile-tar} package/index.js
-      mv index.js libexec/yarnpkg-lockfile.js
-      cp ${./.}/*.js libexec/
-      patchShebangs libexec
+      mv index.js $out/libexec/yarnpkg-lockfile.js
+      cp ${./.}/common.js ${./.}/index.js $out/libexec/
+
+      patchShebangs $out/libexec
+      makeWrapper $out/libexec/index.js $out/bin/prefetch-yarn-deps \
+        --prefix PATH : ${lib.makeBinPath [ coreutils nix-prefetch-git nix ]}
 
-      runHook postBuild
+      runHook postInstall
     '';
 
+    passthru = { inherit tests; };
+  };
+
+  fixup-yarn-lock = stdenv.mkDerivation {
+    name = "fixup-yarn-lock";
+
+    dontUnpack = true;
+    dontBuild = true;
+
+    nativeBuildInputs = [ makeWrapper ];
+    buildInputs = [ nodejs-slim ];
+
     installPhase = ''
       runHook preInstall
 
-      mkdir -p $out/bin
-      cp -r libexec $out
-      makeWrapper $out/libexec/index.js $out/bin/prefetch-yarn-deps \
-        --prefix PATH : ${lib.makeBinPath [ coreutils nix-prefetch-git nix ]}
+      mkdir -p $out/bin $out/libexec
+
+      tar --strip-components=1 -xf ${yarnpkg-lockfile-tar} package/index.js
+      mv index.js $out/libexec/yarnpkg-lockfile.js
+      cp ${./.}/common.js ${./.}/fixup.js $out/libexec/
+
+      patchShebangs $out/libexec
       makeWrapper $out/libexec/fixup.js $out/bin/fixup-yarn-lock
 
       runHook postInstall

pkgs/top-level/all-packages.nix

···
   fetchpijul = callPackage ../build-support/fetchpijul { };
 
   inherit (callPackages ../build-support/node/fetch-yarn-deps { })
+    fixup-yarn-lock
     prefetch-yarn-deps
     fetchYarnDeps;