GitLab

Return an error when a user passes -o and -buildmode=exe to build a package
without a main.

Fixes #20017.

Change-Id: I07d49c75e7088a96f00afe18c9faa842c5d71afb
Reviewed-on: https://go-review.googlesource.com/49371


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>