cmd/go: add ImportMap to go list package struct

cmd/go: add ImportMap to go list package struct

Also populate Imports for test main with go list -test.

Update comment in internal/load/test.go about
p.Imports, p.Internal.RawImports, and p.Imports
being perfectly aligned. The first two are,
but the third is not, as evidenced by CL 111175.

Since p.Imports is not aligned, don't assume that anymore.

Fixes #25949.

Change-Id: Icbfbc881bc01d1e195a759648fbd1c978ddbc161
Reviewed-on: https://go-review.googlesource.com/122878
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>