Remove Puppet::Util::Cacher use from Puppet::Indirector::Indirection

Previously, indirections were storing their termini in a cached_attr, so that
they could be easily cleared for tests. Because this provides no value outside
of testing, we instead simply create an attr_reader for termini, and expire
them manually in tests.

Reviewed-By: Jacob Helwig <jacob@puppetlabs.com>