cmd/go: make hints in error messages more consistent

* All commands the user can run to fix the problem now appear alone on
  a separate line after a tab.
* Removed -d from 'go get' commands.
* Replaced 'go mod tidy' with 'go mod download $modpath' when a
  package might be provided by a module missing a sum.
* Errors about 'path@version' syntax are more explicit.

Fixes #29415
Fixes #42087
Fixes #43430
Fixes #43523

Change-Id: I4427c2c4506a727a2c727d652fd2d506bb134d3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/282121


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>