cmd/go: add go list -cgo and -export

cmd/go: add go list -cgo and -export

We want package analysis tools to be able to ask cmd/go for
cgo-translated files and for the names of files holding export
type information, instead of reinventing that logic themselves.
Allow them to do so, with the new list -cgo and -export flags.

Change-Id: I860df530d8dcc130f1f328413381b5664cc81c3b
Reviewed-on: https://go-review.googlesource.com/108156
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>