This change eliminates our need to guess what the package under test is
in gopls, since `go list` always knows the answer.
Change-Id: I16e482ed3b452bd57cd478b1f8280fcea56474d3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/215020
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
