Settings form takes forever to load when there are many books

I chatted a bit with @Cottser about possible solutions to this. Given that...

1. The status of 📌 [Policy] Move book to contrib in Drupal 11 Fixed suggests it's likely that the book module will be removed from core;
2. As of 2022-03-22, the book module is only used by ~4.5% of sites 🌱 By default deprecate non-experimental modules that are used by less 5% of sites before the next major version Active ;
3. This module is still in beta (i.e.: and could have larger changes) → ; and;
4. Other options seem like a LOT more work (e.g.: modifying the sitemap page to add a way to enable/disable the Book plugin and BookSitemapDeriver);

... we think that we should move the Book plugin and BookSitemapDeriver into a submodule, sitemap_book.

To make this less disruptive, it doesn't seem too difficult to write an update hook that would enable the sitemap_book module on sites where at least one book plugin is already enabled.

Here is a patch. Reviews welcome!

I should add...

dependencies:
  - sitemap:sitemap

... to the sitemap_book module.

Here's a patch with dependencies set properly.

This was done in 📌 Drupal 11 compatibility fixes for sitemap Fixed (although a portion of it has been moved to ✨ Split sitemap_book config schema from sitemap schema Active ), so I'm closing this as a duplicate of 3454907.