Implement nixos-modules/static-page test

lib/default.nix

@@ -1,18 +1,26 @@
 { pkgs, ... }:
 let
-  lib = pkgs.lib;
-  foldersWithNix =
-    path:
-    let
-      folders = lib.attrNames (lib.filterAttrs (n: t: t == "directory") (builtins.readDir path));
-      isFolderWithDefaultNix = folder: lib.pathExists (lib.path.append path "./${folder}/default.nix");
-    in
-    lib.filter isFolderWithDefaultNix folders;
+  inherit (pkgs.lib)
+    attrNames
+    filterAttrs
+    filter
+    pathExists
+    path
+    ;
+  # Get a list of all subdirectories of a directory.
+  getSubDirs = base: attrNames (filterAttrs (n: t: t == "directory") (builtins.readDir base));
+  # Check if a folder with a base path and folder name contains a file with a specific name
+  isFolderWithFile =
+    fileName: basePath: folderName:
+    (pathExists (path.append basePath "./${folderName}/${fileName}"));
+  # Get a list of subfolders that contain a default.nix file.
+  foldersWithNix = base: filter (isFolderWithFile "default.nix" base) (getSubDirs base);
 
 in
 {
-  inherit foldersWithNix;
+  inherit getSubDirs isFolderWithFile foldersWithNix;
 
+  # Get a list of default.nix files that are nix submodules of the current folder.
   loadSubmodulesFrom =
-    path: map (folder: lib.path.append path "./${folder}/default.nix") (foldersWithNix path);
+    basePath: map (folder: path.append basePath "./${folder}/default.nix") (foldersWithNix basePath);
 }