Is your feature request related to a problem? Please describe.
It's currently not possible to efficiently import a path to the store while not including empty directories or directories that would be filtered out. The only way to make it work at all is using recursive readDir calls as done in e.g. NixOS/nixpkgs#188301 and my recent file set combinator work in NixOS/nixpkgs#222981 and NixOS/nixpkgs#245623. Supporting this would also address #4585.

This issue is sponsored by Antithesis ✨

Describe the solution you'd like
A way to control whether empty directories end up in the result of builtins.path:

builtins.path {
  # Whether to include a directory that's empty or fully filtered out, defaulting to true for backwards compat
  includeEmptyDir = path: <bool>;
}

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