cmd/go: clarify error when package is removed in a module

If no module in the build list provides an imported package, we
try to upgrade to the "@latest" version. If there is a requirement on
a version of the module which is newer than the "@latest" version
(e.g., a prerelease or pseudoversion), we cannot upgrade further.

We previously reported "looping trying to add package" when we saw the
package in "@latest" but it was removed later. The meaning of this is
unclear for users, so with this change, we explain the package was
removed.

Fixes #30394

Change-Id: I1b7fec2c37e762fb600e66ee8a4df4aeaf13e67a
Reviewed-on: https://go-review.googlesource.com/c/go/+/169720


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>