Fix flakes follow symlinks

[stevalkr]

Motivation

This is a port of #9499 by @con-f-use with tests requested by @roberth

• symlink points to flake
• symlink points to directory with a subdir that's a flake (ie the symlink isn't the final component but in between)
• symlink points to a directory in a repo
• symlink points from a repo to another directory. Needs --impure? Also test the error.

The last point should be discussed. The current behavior is nix respects the original flake that symlink is pointing to, no --impure needed. This is IMO the desired behavior that I can store links of all the flakes in each project in one place, like how nix manages profiles.

But if someone wants to have one general flake that applies to multiple projects, and for example, uses relative path to files, it's not the case. But I think path: should be the solution.

Context

Closes: #9463
Closes: #9499

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[con-f-use]

You could have added that as a commit to my PR, but I'm fine with yours being merged. If my changes weren't a one-liner, I'd care more about proper attribution, just fyi.

[stevalkr]

Hey @con-f-use, sorry I’m not so familiar with GitHub. It's off topic but do you mean I can clone your fork and push directly to it? There were a lot of commits between then and now, so I’d have to rebase master and force push a commit to your branch.

[con-f-use]

Depends, if you're a maintainer of nix, you can just push to my branch, because I have "Allow edits by maintainers" checked.

It's off topic but do you mean I can clone your fork and push directly to it? There were a lot of commits between then and now, so I’d have to rebase master and force push a commit to your branch.

Yes, kinda. You can make a PR to my forked branch. You don't have to force push or care about the commits that have happened in the mean time if you simply add a new commit that just contains the tests. I.e. just adding the meson.build and symlink-paths.sh changes in a separate commit. You'll have to fork my fork, I think and then do a PR to that.

[roberth]

Alternatively, if you include the tip of @con-f-use's branch in this PR's history, and we merge this, then GitHub will correctly consider both PRs to be merged.
You could for example rebase your branch onto theirs to achieve this.

[stevalkr]

Thanks a bunch, that’s cool to know!