Vendor folder in test breaks dep

[theCarlG]

I'm unable to use dep when I have ffjson as a dependency in a project, because when I try to run dep init it complains about the dependency github.com/foo/vendored, and the reason for this is because dep is trying to flatten vendoring folders found in a dependency, so when it scans ffjson it will find github.com/foo/vendored (in the test folder) and then tries to fetch it from github.