Adding support for recursive file filtering without including empty directories.

[ursi]

I came up with an approach to fix the empty directory problem. I wanted to make you aware of it in case you'd be able to use it to improve the UX. https://github.com/NixOS/nixpkgs/pull/188301.

[zimbatm]

thanks. It looks like a similar idea as https://github.com/numtide/nix-filter/issues/9#issuecomment-1194442257 where the algorithm first traverses the filesystem using builtins.readDir, and then applies the filtered result to builtins.path. It's a good idea.