Fix versionned packages

[juliens]

This PR fixes versionned package.

package demo

import "github.com/dep/v1"

type Demo interface {
  Get() dep.Toto
}

Before the PR this code was generated:

import "github.com/dep/v1"

func Get() v1.Toto {
}

After the PR:

import "github.com/dep/v1"

func Get() dep.Toto {
}