(RE-3597) Move invoke_task into Pkg::Util::RakeUtils

This commit moves the invoke_task method into Pkg::Util::RakeUtils,
where it belongs. It also deprecates the old incantation and adds a
require on rake to the rake_utils module, as rake may not have already
been loaded. Other invocations of invoke_task are updated to use the new
method.
(RE-3597) Add load_packaging_tasks method to Pkg::Util::RakeUtils

This commit moves the giant rake task that was packaging.rake into a
method in Pkg::Util::RakeUtils. This will allow the tasks to be loaded
and invoked purely from ruby, without needing to use rake on the command
line. This should allow shipping of projects without using rake.