cmd/go: report implicit cgo inputs in go list -compiled

Tools using go list -compiled expect to see an Imports list
that includes all the imports in CompiledGoFiles.
Make sure the list includes the cgo-generated imports.

Fixes #26136.

Change-Id: I6cfe14063f8edfe65a7af37522c7551272115b82
Reviewed-on: https://go-review.googlesource.com/128935


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>