Fix using local stack snapshots

[kirelagin]

Used to be part of #334. I added a test for this and made a bunch of little tweaks here and there to make sure that it actually works 🙄.

The newly added test now passes, but, for some reason, it produces the following warnings, I have no clue why:

substituteStream(): WARNING: pattern '/nix/store/<hash>-stack-local-resolver' doesn't match anything in file '/nix/store/<hash>-stack-to-nix-pkgs/stack-local-resolver.nix'
substituteStream(): WARNING: pattern '/nix/store/<hash>-stack-local-resolver' doesn't match anything in file '/nix/store/<hash>-stack-to-nix-pkgs/default.nix'
substituteStream(): WARNING: pattern '/nix/store/<hash>-stack-local-resolver' doesn't match anything in file '/nix/store/<hash.-stack-to-nix-pkgs/pkgs.nix'

cc @balsoft

[kirelagin]

builder for '/nix/store/zbq43r5jdbb4i48j8n15cd0b1yg1488l-stack-simple-0.1.0.0-lib-stack-simple.drv' failed with exit code 1; last 4 log lines:
  unpacking sources
  unpacking source archive /
  cp: '/' and '/' are the same file
  do not know how to unpack source archive /

cool story

[kirelagin]

The problem was that stack-to-nix sets src to path to stack.yaml relative to out and the previous version of the change set stack-yaml=stack.yaml, that is relative. Now it is absolute again, so src gets set correctly, but... it still doesn’t quite work 🤔.

[kirelagin]

Ok, got it, scratch these changes. The correct fix is to make stack-to-yaml mimic stack’s behaviour in that local snapshot is resolved relative to the location of stack.yaml.

[kirelagin]

So I removed the “fix” commit from this PR, but the test is still useful (merge it after bumping the version of nix-tools used by haskell.nix).

[angerman]

awesome! A test case 🎉 !