flake: Move helper derivations in a separate directory (#544)

helpers/rust-analyzer/default.nix

@@ -0,0 +1,39 @@
+{
+  stdenv,
+  python3,
+  rust-analyzer,
+  alejandra,
+}: let
+  extract = stdenv.mkDerivation {
+    pname = "extract_rust_analyzer";
+    version = "1";
+
+    src = ./extract.py;
+
+    dontUnpack = true;
+    dontBuild = true;
+
+    buildInputs = [python3];
+
+    installPhase = ''
+      mkdir -p $out/bin
+      cp $src $out/bin/extract_rust_analyzer.py
+    '';
+  };
+in
+  stdenv.mkDerivation {
+    pname = "rust-analyzer-config";
+    inherit (rust-analyzer) version src;
+
+    nativeBuildInputs = [alejandra extract];
+
+    buildPhase = ''
+      extract_rust_analyzer.py editors/code/package.json |
+        alejandra --quiet > rust-analyzer-config.nix
+    '';
+
+    installPhase = ''
+      mkdir -p $out/share
+      cp rust-analyzer-config.nix $out/share
+    '';
+  }