…ports in 'go mod tidy'

We don't normally keep explicit requirements for test dependencies of
packages loaded from other modules when the required version is
already the selected version in the module graph. However, in some
cases we may need to keep an explicit requirement in order to make use
of lazy module loading to disambiguate an otherwise-ambiguous import.

Note that there is no Go version guard for this change: in the cases
where the behavior of 'go mod tidy' has changed, previous versions of
Go would produce go.mod files that break successive calls to
'go mod tidy'. Given that, I suspect that any existing user in the
wild affected by this bug either already has a workaround in place
using redundant import statements (in which case the change does not
affect them) or is running 'go mod tidy -e' to force past the error
(in which case a change in behavior to a non-error should not be
surprising).

Updates #60313.
Fixes #60352.

Change-Id: Idf294f72cbe3904b871290d79e4493595a0c7bfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/496635
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 2ed6a54)
Reviewed-on: https://go-review.googlesource.com/c/go/+/499635
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>