Return null as pkg for core modules

Hello, thanks for everything you share! This pull request is about a small issue that I have when combining "resolve" with "thunkify" and generators.

```javascript
var resolve = thunkify(require('resolve'));

function* gen() {
  // I want to write
  let [filePath, pkg] = yield resolve(modulePath, opts);

  // But when the `modulePath` is a core module's name (eg. "path"), I have to write
  let res = yield resolve(modulePath, opts);
  let filePath = Array.isArray(res) ? res[0] : res;
  let pkg = Array.isArray(res) ? res[1] : null;
}