libgo: add "vendor" to pkgpath for vendored standard packages

    
    Ensure that the packages vendored into the standard library do not
    have the same pkgpath as the actual packages.  If we don't, attempts
    to build and test the actual packages will get confused.  The specific
    error I was seeing was import loops, causing some of the packages to
    fail to get initialized, causing an obscure run time crash.
    
    Reviewed-on: https://go-review.googlesource.com/43610

From-SVN: r248168