all: embrace unit packages where possible

flake.nix

@@ -15,10 +15,28 @@
           pkgs = import nixpkgs { inherit system; };
         in
         {
+          # nixos/rfcs#140
+          # Only produces the package set of the proposed functionality.
+          # Unstable names are variables.
           packages =
-            lib.filterAttrs
-              (_: drv: builtins.elem system (drv.meta.platforms or [ ]))
-              (import ./pkgs/top-level/all-packages.nix { inherit pkgs; });
+            let
+              unitDir = "unit";
+              packageFun = "package.nix";
+
+              callUnitRoot = root:
+                let
+                  shards = lib.attrNames (builtins.readDir root);
+                  namesForShard = shard: lib.mapAttrs'
+                    (name: _: { inherit name; value = "${root}/${shard}/${name}"; })
+                    (builtins.readDir "${root}/${shard}");
+                  namesToPath = lib.foldl' lib.recursiveUpdate { } (map namesForShard shards);
+                  units = lib.mapAttrs (_: path: pkgs.callPackage "${path}/${packageFun}" { }) namesToPath;
+                in
+                units;
+              legacyPackages = import ./pkgs/top-level/all-packages.nix { inherit pkgs; };
+              onlyAvailable = lib.filterAttrs (_: drv: builtins.elem system (drv.meta.platforms or [ ]));
+            in
+            onlyAvailable (legacyPackages // callUnitRoot "${./pkgs}/${unitDir}");
 
           checks = {
             nixpkgs-fmt = pkgs.writeShellScriptBin "nixpkgs-fmt-check" ''