cmd/go: emit go list error for local non-existant packages

In CL 129061, a check was added for patterns that reference
nonexistent local directories. While this prevented unnecessary
network lookups (fixing #26874), it caused "go list -e" to exit with
an error instead of listing packages with error messages.

This change avoids the network lookup and does not exit for these
kinds of packages. Errors are still reported by
internal/load.LoadImport for packages that don't exist.

Fixes #28023

Change-Id: I0a648269e437aed3a95bfb05461a397264f3793f
Reviewed-on: https://go-review.googlesource.com/c/151800


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