refactor in style of synaptic standard

util/recursively-import.nix

@@ -0,0 +1,21 @@
+{ lib }:
+let
+  recurseIntoFolders =
+    elem:
+    if lib.isPath elem && lib.pathIsDirectory elem then
+      lib.filesystem.listFilesRecursive elem
+    else
+      # If it's not a folder, return it unchanged. This handles single-files and
+      # literal modules (written with {} syntax)
+      lib.singleton elem;
+
+  filterNixFiles =
+    paths:
+    builtins.filter
+      # filter the files for `.nix` files. if it's not a file, it can stay.
+      (path: !builtins.isPath path || lib.hasSuffix ".nix" path)
+      # Expand any folders into all the files within them. Note that this comes
+      # BEFORE the filtering that's happening above
+      (builtins.concatMap recurseIntoFolders paths);
+in
+filterNixFiles