go/types: use new importer.ForCompiler for gotype command
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 <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

josharian/go,CAFxX/go,golang/go,golang/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,golang/go,akutz/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,golang/go,josharian/go,josharian/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,akutz/go,josharian/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,josharian/go,josharian/go,akutz/go,josharian/go,cloudflare/go