cmd/go: clean up handling of go mod tidy -go=v during modload

cmd/go: clean up handling of go mod tidy -go=v during modload

Slight cleanup of the special case for "mod tidy" during modload
of a module with no go version. Now it's clearer what is special.

For #57001.

Change-Id: I4a3ad0a948ccb5c62365f2a7f7f7de5582cc57c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/499535
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>