Registers an extension in runtime registries for execution.
string $type: The extension type; e.g., 'module' or 'theme'.
string $name: The internal name of the extension; e.g., 'node'.
string $uri: The relative URI of the primary extension file; e.g., 'core/modules/node/node.module'.
function system_register($type, $name, $uri) {
drupal_get_filename($type, $name, $uri);
drupal_classloader_register($name, dirname($uri));
}