cmd/go: set Incomplete field on go list output if no files match embed

cmd/go: set Incomplete field on go list output if no files match embed

If no files match the embed pattern, the Error field will be set on
the package output by go list. Also set the Incomplete field for
consistency.

Fixes #43727

Change-Id: I5b4bb2a03a751269641a9bc4ef1d0fa0e37d46aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/284257
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>