cmd/vet: provide access to externally defined types
cmd/vet: provide access to externally defined types

Preliminaries for fixing issue 6212.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/145370043

d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools