MDL-81919 core: Add support for autoloading legacy classes

lib/classes/component.php

@@ -690,17 +690,20 @@ $cache = ' . var_export($cache, true) . ';
         self::$classmap = [];
 
         self::load_classes('core', "$CFG->dirroot/lib/classes");
+        self::load_legacy_classes($CFG->libdir);
 
         foreach (self::$subsystems as $subsystem => $fulldir) {
             if (!$fulldir) {
                 continue;
             }
             self::load_classes('core_' . $subsystem, "$fulldir/classes");
+            self::load_legacy_classes($fulldir);
         }
 
         foreach (self::$plugins as $plugintype => $plugins) {
             foreach ($plugins as $pluginname => $fulldir) {
                 self::load_classes($plugintype . '_' . $pluginname, "$fulldir/classes");
+                self::load_legacy_classes($fulldir);
             }
         }
         ksort(self::$classmap);
@@ -1401,6 +1404,34 @@ $cache = ' . var_export($cache, true) . ';
         }
     }
 
+    /**
+     * Load legacy classes based upon the db/legacyclasses.php file.
+     *
+     * The legacyclasses.php should contain a key => value array ($legacyclasses) where the key is the class name,
+     * and the value is the path to the class file within the relative ../classes/ directory.
+     *
+     * @param string|null $fulldir The directory to the legacy classes.
+     */
+    protected static function load_legacy_classes(?string $fulldir): void {
+        if (is_null($fulldir)) {
+            return;
+        }
+
+        $file = $fulldir . '/db/legacyclasses.php';
+        if (is_readable($file)) {
+            $legacyclasses = null;
+            require($file);
+            if (is_array($legacyclasses)) {
+                foreach ($legacyclasses as $classname => $path) {
+                    $fullpath = "{$fulldir}/classes/{$path}";
+                    if (file_exists($fullpath)) {
+                        self::$classmap[$classname] = $fullpath;
+                    }
+                }
+            }
+        }
+    }
+
     /**
      * Returns a list of frankenstyle component names and their paths, for all components (plugins and subsystems).
      *