hook_library() and hook_library_alter() have been renamed to hook_library_info() and hook_library_info_alter()

hook_library() and hook_library_alter() were not named according to standard Drupal naming patterns. These hooks have been renamed as hook_library_info() and hook_library_info_alter() respectively.

Backdrop inherited this from a change in the early Drupal 8 cycle. See https://www.drupal.org/node/1286868.