go/types: add lookup method to context and factor out LookupParent calls

go/types: add lookup method to context and factor out LookupParent calls

R=go1.11

Also: Moved Checker.pos field into context where it belongs.

This is a cleanup/code factoring.

For #22992.

Change-Id: If9d4f0af537cb181f73735e709ebc8258b2a1378
Reviewed-on: https://go-review.googlesource.com/83017
Reviewed-by: Alan Donovan <adonovan@google.com>