Skip local dependencies in generateNix

[albertov]

Without this fix node will throw an error when it runs generateNix on a local dependency:

TypeError: Cannot read property 'split' of undefined
    at generateNix (/nix/store/115k8bis4wsqyrxspx0lyjb60z5ak2vz-yarn2nix-1.0.0/libexec/yarn2nix/deps/yarn2nix/bin/yarn2nix.js:42:39)
    at Promise.all.then (/nix/store/115k8bis4wsqyrxspx0lyjb60z5ak2vz-yarn2nix-1.0.0/libexec/yarn2nix/deps/yarn2nix/bin/yarn2nix.js:142:5)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
    at Function.Module.runMain (module.js:695:11)
    at startup (bootstrap_node.js:191:16)
    at bootstrap_node.js:612:3

[zimbatm]

thanks!