cmd/vet: adjust vet to use go/types and friends from std repo

cmd/vet: adjust vet to use go/types and friends from std repo

- s|"golang.org/x/tools/go/exact"|"go/constant"|
- s|"golang.org/x/tools/go/types"|"go/types"|
- removed import of gcimporter
- import "go/importer" instead
- trivial adjustments to make use of go/importer
- adjusted import paths for whitelist.go

Change-Id: I43488ff44c329cd869c92dcc31193fb31bebfd29
Reviewed-on: https://go-review.googlesource.com/10695
Reviewed-by: Rob Pike <r@golang.org>