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.