These shouldn't need to be accessed from outside the asset system
itself, so they should not be exposed with its API. Move them to an own
asset system private headers. This is a further step towards making
asset system classes more encapuslated, so behavior and data flow can be
controlled better (which helps addressing threat safety issues).
Personally I've found it quite confusing to work on higher level issues
of the asset catalog system, because I got lost in the multiple classes.
Hopefully separating them more clearly helps with that too.
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