use !defined to conditionally install prereqs

#27 

on further research:

this was a known potential issue and was fixed here: https://github.com/gini/puppet-archive/commit/610e943d0c8b60c01788335b74920b11b95fe201

but it was later changed to this "flexible" version: https://github.com/gini/puppet-archive/commit/0f133ab5629a6adf2030e943371321ba642fe634

using `$dependency_class` is novel, but `archive` is a defined type, not a class: I can't override that value with automatic hiera binding.

I'm running this branch in my lab environment right now and it seems to solve my issues with `gini/archive` and `bfraser/grafana`.