(#5428) More fully "stub" Puppet::Resource::Reference for use with storedconfigs

The Puppet::Resource::Reference class wasn't stubbing enough of the 0.25.x
behavior to satisfy the needs of storedconfigs. Since P::R::Reference,
and Puppet::Resource were merged as part of 2.6.x, we can pretend that
P::Resource is P::R::Reference for the purposes of loading data from
storedconfigs. This should still satisfy the over-the-wire serialization
needs of 0.25.x.

This also changes internal references to @parameters in
Puppet::Resource(::Reference) to go through a parameters method. This
allows us to "initialize" this instance variable lazily, since loading via
YAML bypasses the normal initialize method.

Paired-with: Daniel Pittman <daniel@puppetlabs.com>
Reviewed-by: Markus Roberts <markus@puppetlabs.com>