Support for Yarn workspaces

[nightkr]

This adds a new argument workspaceDependencies to mkYarnPackage, the contents of which are added to a Yarn Workspace before yarn install. Example:

  rec {
    package-one = yarn2nix.mkYarnPackage {
      src = ./workspace/package-one;
      yarnLock = ./workspace/yarn.lock;
      workspaceDependencies = { inherit package-two; };
    };
    package-two = yarn2nix.mkYarnPackage {
      src = ./workspace/package-two;
      yarnLock = ./workspace/yarn.lock;
    };
  }.package-one

Notes:

• Workspace dependencies must be manually added to workspaceDependencies
  • However, transitive dependencies are supported (for a relationship a -> b -> c, a does not need to explicitly depend on c)
• Local dependency build steps are not supported
  • This is because local dependencies are removed at the end of mkYarnModules and added back in mkYarnPackage, to avoid re-running yarn after source-only changes

This (sort of) fixes #57.

[nightkr]

Done @zimbatm

[zimbatm]

thanks!