cmd/go: don't print 'go get' deprecation notices in the main module

If a user runs 'go get example.com/cmd' for a package in the main
module, it's more likely they intend to fill in missing dependencies
for that package (especially with -u). If the intent were only to
build and install, 'go install example.com/cmd' would be a better
choice.

For #43684

Resolving a comment on CL 305670.

Change-Id: I5c80ffdcdb3425b448f2f49cc20b07a18cb2bbe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/318570


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