Dynamically register layouts in hook_theme() or not

Modules can provide layouts too (that's pretty much the point of decoupling layouts from themes), and currently the theme registry builder has no auto-discovery of suggestions from modules, so modules need to declare all of the suggestions they implement anyway.

However, given some of what is being attempted in Theme Component Library, we may want to rethink and optimize lots of our current implementation of how suggestions integrate with the theme registry.

Also, for layouts specifically, per #1787846-99: Themes should declare their layouts, I'll open an issue for discussing whether we want to theme_render_template() layouts directly and skip all of theme() entirely, in which case, we won't need them in hook_theme() or the theme registry at all.

The layout module has been removed from core.