allow using local dependencies by providing their paths

[icewind1991]

Provide a solution for #98

With this patch, given a package.json with

"dependencies": {
    "foo": "file:../foo"
},

you can tell npmlock2nix to resolve the local dependency using

npmlock2nix.v2.node_modules {
  src = ./.;
  localPackages = {
    "foo" = ../foo;
  };
};

[andir]

Could you add a test case for this so we don't break it?

[icewind1991]

added some tests

[c0deaddict]

can this be merged? i would like to use npmlock2nix with a phoenix project.