ActiveSupport::Dependencies
参考：
ActiveSupport::Dependencies | Ruby on Rails [Official]
rails/dependencies.rb at 5-2-stable · rails/rails – GitHub
require_dependency
参考：
ActiveSupport::Dependencies::Loadable | Ruby on Rails [Official]
rails/dependencies.rb at v5.2.3 · rails/rails – GitHub
require_or_load
参考：
require_or_load – rails/dependencies.rb at v5.2.3 · rails/rails – GitHub
autoloaded_constants
参考：
autoloaded_constants – rails/dependencies.rb at v5.2.3 · rails/rails – GitHub