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: https://github.com/cdepillabout/stacklock2nix/blob/22676dfc45fa1c33899ba1da1a23665172a18ba7/nix/build-support/stacklock2nix/default.nix#L390

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.)