This makes the backtrace for a deprecated icon name error clearer in
terms of pointing out which plugin the icon is being registered from.
The error will only be raised in the test environment.
This PR also extracts out to a separate module the hashmaps & logic for
converting FA4 era icons to discourse FA6 compatible icons. This
isolates that logic for reuse in DiscoursePluginRegistry - otherwise
pulling in the whole SvgSprite resulted in errors.
