To make the error fix more useful, expand typecheck to gather
more information about struct fields, typecheck range statements,
typecheck indirect and index of named types, and collect information
about assignment conversions.

Also, change addImport to rename top-level uses of a to-be-imported
identifier to avoid conflicts.  This duplicated some of the code in
the url fix, so that fix is now shorter.

R=iant, r, r
CC=golang-dev
https://golang.org/cl/5305066