goimports not importing internal packages when working outside of GOPATH

[cgilling]

I was recently debugging a problem with goimports not working properly and found it wasn't working quite right when working with modules outside of the gopath, specifically importing internal packages within your module. I created a quick fix for this for anyone who has this problem as well. (I haven't done a PR because I think the problem is actually in goimports code, but I got it working and haven't had the time to dig into goimports code)