The check was previously disallowing package main from even importing
a non-function symbol named "main".

Fixes #24801.

Change-Id: I849b9713890429f0a16860ef16b5dc7e970d04a4
Reviewed-on: https://go-review.googlesource.com/106120
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>