Support wp-db.php in WP 4.9 and older releases in maybe_load_wpdb

Author: swissspidy

src/DB_Command.php

@@ -2473,13 +2473,15 @@ protected function maybe_load_wpdb() {
 			define( 'WP_CONTENT_DIR', ABSPATH . 'wp-content' );
 		}
 
+		$wpdb_file_name = file_exists( ABSPATH . WPINC . '/class-wpdb.php' ) ? 'class-wpdb.php' : 'wp-db.php';
+
 		// Load prerequisite WordPress files if not already loaded.
 		$prereqs = [
 			'load.php'           => ABSPATH . WPINC . '/load.php',
 			'compat.php'         => ABSPATH . WPINC . '/compat.php',
 			'plugin.php'         => ABSPATH . WPINC . '/plugin.php',
 			'class-wp-error.php' => ABSPATH . WPINC . '/class-wp-error.php',
-			'class-wpdb.php'     => ABSPATH . WPINC . '/class-wpdb.php',
+			$wpdb_file_name      => ABSPATH . WPINC . '/' . $wpdb_file_name,
 		];
 
 		foreach ( $prereqs as $name => $path ) {