output: allow to have host defined in subdirectory

outputs.nix

@@ -22,22 +22,22 @@ self = {
     filePaths = lib.pipe ./hosts [
       builtins.readDir
       ( lib.filterAttrs (name: type:
-        # filter out non-nix files
-        type == "regular"
-        # filter out files that don't end in .nix
-        && lib.hasSuffix ".nix" name
+        ( # regular .nix files
+          (type == "regular" && lib.hasSuffix ".nix" name)
+          || # directories that contain a default.nix file
+          (type == "directory" && builtins.pathExists "${./hosts}/${name}/default.nix")
+        )
         # filter out files that start with .
         && !lib.hasPrefix "." name
       ))
-      (lib.mapAttrsToList (name: _: name))
     ];
     nixosSystem = import "${inputs.nixpkgs}/nixos/lib/eval-config.nix";
   in
     # mapped list of nix file paths to attrSet with initialized NixOS configurations,
     # whose names are derived from file names
     lib.pipe filePaths [
-      (builtins.map (name: {
-        name = builtins.substring 0 (builtins.stringLength name - 4) name;
+      (builtins.mapAttrs (name: type: {
+        name = if type == "directory" then name else builtins.substring 0 (builtins.stringLength name - 4) name;
         value = nixosSystem {
           inherit lib;
           modules = [
@@ -53,6 +53,7 @@ self = {
           specialArgs = { inherit self inputs; };
         };
       }))
+      builtins.attrValues
       builtins.listToAttrs
     ];
 };