Motivation for this change

package.json files can specify local paths as dependencies, and yarn creates corresponding entries in the yarn.lock file for them. Currently yarn2nix errors when processing those entries, but still exits with a zero status. This means that yarn2nix.mkYarnPackage writes an incomplete and unparseable nix file to the store and then tries to read it, with an error like error: syntax error, unexpected $end, at /nix/store/76ml5lgvrc41b5c4y8jsy1iw8z50cw8s-yarn.nix:3081:6.

One way to reproduce:

$ git clone git@github.com:yarnpkg/yarn.git
# ...
$ cd yarn 
$ nix run -f channel:nixos-unstable yarn2nix -c yarn2nix
# ...
    {
      name = "eslint-plugin-relay-0.0.21.tgz";
      path = fetchurl {
        name = "eslint-plugin-relay-0.0.21.tgz";
        url  = "https://registry.yarnpkg.com/eslint-plugin-relay/-/eslint-plugin-relay-0.0.21.tgz";
        sha1 = "fbda01bd783aa7ed9eb0effe410049911e1449ef";
      };
    }
(node:62298) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): TypeError: Cannot read property 'split' of undefined
$ echo $?
0

I don't know what the policy around the yarn2nix version number is; sorry if I was wrong to bump it.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.