[amberctl] Mirror source edits to pkg_resolver

To ease the migration from using amber source configs to using
pkg_resolver repository configs, this change modifies the source
management commands in amberctl to perform roughly equivalent operations
on the RepositoryManager and rewrite.Engine.

Test: modified amberctl test to verify pkg_resolver state

PKG-704 #done

Change-Id: I92853a61b59a60693014853862a9494c9d227caa