cmd/go: print deprecation notice for 'go get cmd'

The notice is shown when 'go get' is invoked with the -d flag, and
the arguments match at least one main package.

This reverts CL 274552.

For #43684

Change-Id: I42e6731455f22988bf72dde1d5a76d197e9e3954
Reviewed-on: https://go-review.googlesource.com/c/go/+/305670


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>