Import Danger::Helper as internal plugin

What does this MR do and why?

Add missing Danger::Helper import

Since we are now accessing Config in the Engine initializer, we are accessing Danger::Helper before we import all plugins, so we need to require it.

To that end, we now split internal from ordinary plugins and require internal plugins in the Engine initializer.

Fixes https://gitlab.com/gitlab-org/labkit-ruby/-/jobs/1837131511

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