introduce importDir

avoid repeating myself

to create a new image, create a new folder under ./images that contains
a default.nix. That's it.

lib/importDir.nix

@@ -0,0 +1,21 @@
+{ lib }:
+
+importFn: baseDir:
+let
+  dirEntries =
+    builtins.attrNames
+      (lib.filterAttrs
+        (k: v: v == "directory")
+        (builtins.readDir baseDir));
+
+  absDirs =
+    builtins.map
+      (dir: "${toString baseDir}/${dir}")
+      dirEntries;
+
+  imports =
+    builtins.map
+      (dir: { name = builtins.baseNameOf dir; value = importFn dir; })
+      absDirs;
+in
+  builtins.listToAttrs imports