cmd/go: make module suggestion more friendly

We are trying to avoid by not automatically updating go.mod. The
suggestion should be that users actually add the dependencies they
need, and the command in an easily copy-pastable form now.

Fixes: #43430

Change-Id: I2227dab498fcd8d66184c94ebe9e776629ccadfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/280713


Run-TryBot: Baokun Lee <bk@golangcn.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>