Fix 'off by one error' with directory copy

The cp -r <pkgdir>/ imp creates a duplicate directory, e.g.,

cd ~/go/local/src/github.com/xyzzy/deps
goven github.com/kr/goven

creates ~/go/local/src/github.com/xyzzy/deps/github.com/kr/goven/goven not
~/go/local/src/github.com/xyzzy/deps/github.com/kr/goven as desired.