Also: make it easier to find packages. Hide the importMap field
(never used) and expose a (*Program).Package method which searches
importMap and then Created.
Also: move huge comments into doc.go.
Change-Id: Iad96a12524b7c41ad9acd1e806af23171e71fa7c
Reviewed-on: https://go-review.googlesource.com/9030
Reviewed-by: Robert Griesemer <gri@golang.org>
Alan Donovan