[Enhancement]: Make internal/external distinction configuration not code

[azaroth42]

Priority Level

Medium

Background

The distinction is only at the processing level, not inherent to the datasets. It should be possible to build a Getty Vocabularies LUX instance by running all of aat, tgn and ulan, and not yuag, ycba, etc.

Description

Remove .internal/.external (urgh) everywhere (!) and replace with some neutral dataset property, plus run-time configuration for which sources should be processed completely and which should be only referenced into.

Changes Required

1. Work out what the changes are
2. Update this list
3. Make the changes