#1 Properly merge composer extras

src/Plugin.php

@@ -83,7 +83,7 @@ class Plugin implements PluginInterface {
             $extra[$item][$path] = ['type:drupal-' . $type];
           }
         }
-        $extra[$item] += $this->readJson($item, $extra[$key], $io);
+        $extra[$item] = array_merge($extra[$item], $this->readJson($item, $extra[$key], $io));
         $changed = TRUE;
       }
     }