flake: properly handle builtins.readDir attrset and compute pkgNames

flake.nix

@@ -10,13 +10,12 @@
 
     pkgsDir = ./pkgs;
 
-    entries = if builtins.pathExists pkgsDir then builtins.readDir pkgsDir else [];
+    # `builtins.readDir` returns an attribute set of entries. Use an empty
+    # attrset when missing so the type stays consistent during eval.
+    entries = if builtins.pathExists pkgsDir then builtins.readDir pkgsDir else {};
 
-    # builtins.readDir returns a list of names; when pkgs are present as
-    # committed directories use them directly. (If you use git submodules,
-    # their contents are not available during flake evaluation unless
-    # vendored.)
-    pkgNames = entries;
+    # only subdirectories: filter the entries attribute-set and get the names
+    pkgNames = lib.attrNames (lib.filterAttrs (_: t: t == "directory") entries);
 
     loadPkg = name:
       let