cmd/go: provide a better error message when there's no go directive
On Go 1.21+ it's an error for a workspace to contain a module with a
version newer than the workspace's stated go version. If the workspace
doesn't explicitly have a go version it's explicitly 1.18. So if a
workspace without a go directive contains a module whose go directive
is newer on it's always an error for 1.21+. In the error, before this
CL the error would read "module <path> listed in go.work requires go
>= <version>, but go.work lists go 1.18". After this change the second
clause would read "but go.work implicitly requires go 1.18.
Fixes #66207
Change-Id: I44680880162a82e5cee9cfc8655d6774add6f762
Reviewed-on: https://go-review.googlesource.com/c/go/+/570735
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>