Test the api of the internal imports package to make sure that it does
not crash when given nil as opts or Env.
Change-Id: I0127d550a49f63040efb16c07e8cff8b599bbe3c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/190000
Reviewed-by: Heschi Kreinick <heschi@google.com>
