[PP-1] Don't export computed fields

Unless I'm missing something, we definitely cannot allow path_alias entities to be exported, at least not automatically. Why? Because the system path stored in the entity is going to include things like /node/3, which uses a non-portable serial ID.

So that's a dead end.

Therefore, I think the path field on entities needs to be exported as part of the entity. Since that is probably one of the only computed fields that needs to be exported with an entity -- if not the only such field, at least in core -- I would propose we add a normalizer specifically for this purpose to the path_alias module, which momentarily (non-persistently) marks the path field of an entity as being non-computed so it will be exported.