cmd/go/internal/modfile: report error for extra text around version

Fixes #34697

Change-Id: Iedfa3d46d558510f3bd1fdf9466cd974793d9ecd
Reviewed-on: https://go-review.googlesource.com/c/go/+/199017


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>