cmd/go: improve error message for missing import starting with cmd/

In modload.Import, confirm that the import path does not start with
"cmd/" before calling QueryPackage, which returns a less helpful
error.

In load.loadPackageData, don't wrap errors with "unknown import path".
The wrapped error should always include the import path, and it's also
repeated in the PackageError wrapper.

Fixes #31031

Change-Id: I071efa22e3842c62831d096f888a8006811fe724
Reviewed-on: https://go-review.googlesource.com/c/go/+/189157


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