Fix use of $::apt::params::backports and $::apt::params::xfacts.

[Farzy]

This commit fixes the usage of 2 default parameters which were misnamed $::apt::backports and $apt::xfacts and adds missing "inherits apt::params" or "include apt::params".

[daenney]

This goes against everything we've designed this module for. You have to have apt included in your manifests so there is no need to use include apt::params or inherit apt::params anywhere else. These values are directly accessible under the apt:: namespace which is what this code was doing and the tests proved it was working.

@mhaskel HALP 😭