Rename `loader_tree` to `app_page_loader_tree` (#70283)
This is a pure refactoring in preparation for introducing an `app_route_loader_tree` in a future PR.
The `app_page_loader_tree` collects information from the app directory for app _page_ entries, whereas the upcoming `app_route_loader_tree` will collect information from the app directory tailored to app _route_ entries.
