cmd/go/internal/modload: replace import error message from goroot to std

cmd/go/internal/modload: replace import error message from goroot to std

When importing a package that does not exist, it would show goroot error
message and path. We would like to replace goroot with std instead.

Fixes #56965.

Change-Id: I86f8a7fab6555b68f792a3a4686de20d51eced8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/453895
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>