flake: use inputs block, readDir pkgs safely, and builtins.map for packages

flake.nix

@@ -1,5 +1,7 @@
 {
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs";
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs";
+  };
 
   outputs = { self, nixpkgs }:
   let
@@ -8,11 +10,13 @@
 
     pkgsDir = ./pkgs;
 
-    entries = builtins.readDir pkgsDir;
+    entries = if builtins.pathExists pkgsDir then builtins.readDir pkgsDir else [];
 
-    # only subdirectories
+    # builtins.readDir returns a list of names; when pkgs are present as
-    pkgNames =
+    # committed directories use them directly. (If you use git submodules,
-      lib.attrNames (lib.filterAttrs (_: t: t == "directory") entries);
+    # their contents are not available during flake evaluation unless
+    # vendored.)
+    pkgNames = entries;
 
     loadPkg = name:
       let
@@ -23,13 +27,7 @@
         flake.packages.${system}.default;
 
     # build final package set
-    packages =
+    packages = lib.listToAttrs (builtins.map (name: { inherit name; value = loadPkg name; }) pkgNames);
-      lib.listToAttrs (
-        map (name: {
-          inherit name;
-          value = loadPkg name;
-        }) pkgNames
-      );
 
   in {
     packages.${system} = packages;