Running gopack without a directory argument raises lstat error

When running gopack only with a version number inside your module directory, it will try to call [filepath.Walk() with an empty string](https://github.com/cloudsmith-io/gopack/blob/5645b31977bd42af4e8cbb08fcad224c9068fae5/archive/archive.go#L26) which causes a lstat error.

This is easily fixable by checking if dir is an empty string and setting it to "." in root.go.

I will also open a PR for this change for convenience, feel free to accept/close it :)