[release-branch.go1.13] cmd/go/internal/modload: fix swapped paths in error message

Cherry-picked from CL 191997.

Updates #33879
Fixes #33885

Change-Id: Ifc91490b1cb791fdf5ffe69ef81c0ec0e6cbecc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/191997


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
(cherry picked from commit 8f5353fd)
Reviewed-on: https://go-review.googlesource.com/c/go/+/191972
TryBot-Result: Gobot Gobot <gobot@golang.org>