Is your feature request related to a problem? Please describe.
Some registries act wrong when the contents are not obtained from a provider in datagen.

Describe the solution you'd like
Passing the HolderLookup.Provider lets you get the necessary holder. This is provided in DataProvider run in the form of the registries local so it could be passed in BookSubProvider#generate, then being available in generateCategories (perhaps as an overload to avoid breaking changes), which would make it available for categories and entries.