Allow loading cookbooks from the Berksfile.lock file

I needed to promote a cookbook that was not part of the immediate Berksfile, but was in the Berksfile.lock as it was a dependency of a cookbook.
Raise error when no lockfile is found

@sethvargo should we keep track somewhere that this can be solved
differently in master?
Merge pull request #1 from RSO/patch-1

Allow loading cookbooks from the Berksfile.lock file