go/types: use new importer.ForCompiler for gotype command

This will produce better error messages (position information)
for errors referring to imported objects.

Change-Id: I24646ae803e6b8f78e9240310a858d4095e9463d
Reviewed-on: https://go-review.googlesource.com/c/152538


Reviewed-by: Alan Donovan <adonovan@google.com>