gvt doesn't include licenses from nested vendored directories

[casualjim]

The terraform project contains a vendor directory, in there it has a bunch of libraries that contain license files. When vendoring terraform with gvt the license files no longer appear.

original repo:
https://github.com/hashicorp/terraform/tree/master/vendor/github.com/mitchellh/mapstructure

repo with gvt vendored terraform
https://github.com/casualjim/gvtbug/tree/master/vendor/github.com/hashicorp/terraform/vendor/github.com/mitchellh/mapstructure