cmd/go: print hint when 'go install' run without version outside module
If 'go install' is invoked in module mode outside a module with a
package that could only be loaded from a module, it will now suggest
running 'go install pkg@latest'.
'go install' will still work outside a module on packages in std and
cmd, as well as .go files specified on the command line.
Fixes #42638
Change-Id: Ib0963935f028b7656178bc04a279b1114de35fbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/277355
Run-TryBot: Jay Conrod <jayconrod@google.com>
Trust: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>