go/internal/gcimporter: populate (*types.Package).Imports

go/internal/gcimporter: populate (*types.Package).Imports

This is a copy of an upstream change to the tools repo:
https://go-review.googlesource.com/#/c/8924/

This is a second attempt at CL 8954, with the necessary change to
go/build's deps test.

Change-Id: Ib798498cf85fea0baec5667e9324d11f6ae8ad64
Reviewed-on: https://go-review.googlesource.com/9173
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>