flakeref: follow symlinks

[con-f-use]

fixes #9463

Motivation

Given /tmp/flakelink is a symlink to an actual flake directory, currently this happens:

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

This PR attempts to fix that.

Warning: I have no prior knowledge of the nix-codebase or much of a background in C++. Please review carefully. In particular, I do not know if there's any ramification, regarding the use of flakeref in the inputs definitions of flake.nix files or other edge-cases. @infinisil and @YorikSar pressured me into this 😉

Priorities

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[zedseven]

I was about to open a PR with the same change.

This PR, if merged, would fix #8013.

[iFreilicht]

I think this is a good and simple change. One thing I would suggest is adding a functional test for this. Have a look at this section of the manual for info on how to set this up.

[nixos-discourse]

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

https://discourse.nixos.org/t/2024-03-11-nix-team-meeting-132/42960/1

[Mic92]

Added you as a coauthor in https://github.com/NixOS/nix/pull/12286