Approach of copying subdirs for git extra-deps breaks in the presence of relative symlinks

[isomorpheme]

In stack.yaml I have something like:

- git: https://github.com/tensorflow/haskell.git
  commit: 925c2e95151c0ea1592c26b51a5d98e4ca4a2fe7
  subdirs:
  - tensorflow
  - tensorflow-proto

I see that this leads to a different src derivation for each subdir here:

stacklock2nix/nix/build-support/stacklock2nix/default.nix

Line 390 in 22676df

cp -r "${rawSrc}/${haskPkgLock.subdir}" "$out"

But for this repo, that leads to a failing build, because the tensorflow-proto subdir has a relative symlink third_party in it that points to ../third_party - this symlink becomes dangling when only the subdir is copied to the store. (third_party itself happens to have a submodule in it, but that's #44.)

[cdepillabout]

@isomorpheme Thanks for submitting this, and sorry for taking so long to get to this!

I'm not sure what to do here. This just seems like a tricky corner case. Do you have any suggestions on how to handle this?

It feels like to me that a "nice" solution here probably doesn't exist. The best we could do is add some sort of hook that allows the user to go in and manually fix up the repo definition. But at that point, the user could just add their own overlay on top of the overlay produced by stacklock2nix, so the additional hook seems somewhat pointless.

[cdepillabout]

Oh wait, I see that you already have a suggestion in #46! Let me take a look at that