gvt restore doesn't work recursively

[shingo-szk]

Hi.

I tried to use gvt restore to resolve dependency recursively, but it doesnt' work.

In source code, downloadDependency tries to read from the copied tree.

https://github.com/FiloSottile/gvt/blob/50d83ea21cb0405e81efd284951e111b3a68d701/restore.go#L126-L138

But Copypath() seems to ignore manifest because it doesn't look like Go file.

Regards.