bug: nix commands do not follow flake-url symlinks

[con-f-use]

Describe the bug

If the flake path is a symlink, Nix refuses to interact with a given flake "because it's not a directory".

Steps To Reproduce

$ pwd
/tmp

$ mkdir flake

$ ln -s flake flakelink

$ cd flake

$ nix flake init
wrote: /tmp/flake/flake.nix

$ nix build '/tmp/flakelink/#hello'
error: path '/tmp/flakelink' is not a flake (because it's not a directory)

Same for other flake commands.

Expected behavior

The symlink url should exhibit approximately the same behavior as cd ../flakelink && nix build './#hello' would.
I'd expect the symlink is followed and the flake is build, with the result link in the current directory.

$ nix-env --version
nix-env (Nix) 2.18.1

[con-f-use]

Even if that is intended behavior, there it might be possible to have a trailing / in the flake path to indicate to follow symlinks, or a warning when doing so or a --follow-link-to-flake flag for mitigation of this.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-12-01-nix-team-meeting-minutes-108/36402/1