purescm: avoid `lib.fileset`

pkgs/by-name/pu/purescm/README.rst

···
 To update this package
 ======================
 
-#. Bump the ``./package.json`` version pin
-#. Run ``nix-shell -p nodejs --command "npm i --package-lock-only"``
+#. Bump the ``./manifests/package.json`` version pin
+#. Run ``(cd manifests && nix-shell -p nodejs --command "npm i --package-lock-only")``
 #. Update ``npmDeps.hash`` in the ``package.nix``

pkgs/by-name/pu/purescm/package.nix

···
 let
   inherit (lib) fileset;
 
-  packageLock = builtins.fromJSON (builtins.readFile ./package-lock.json);
+  packageLock = builtins.fromJSON (builtins.readFile ./manifests/package-lock.json);
 
   pname = "purescm";
   version = packageLock.packages."node_modules/${pname}".version;
···
   package = buildNpmPackage {
     inherit pname version;
 
-    src = fileset.toSource {
-      root = ./.;
-      fileset = fileset.unions [
-        ./package.json
-        ./package-lock.json
-        ./.gitignore
-      ];
-    };
+    src = ./manifests;
     dontNpmBuild = true;
 
     npmDeps = fetchNpmDeps {
-      src = fileset.toSource {
-        root = ./.;
-        fileset = fileset.unions [
-          ./package-lock.json
-          ./package.json
-        ];
-      };
+      src = ./manifests;
       hash = "sha256-ljeFcLvIET77Q0OR6O5Ok1fGnaxaKaoywpcy2aHq/6o=";
     };