add getter for global composer

src/Composer/Plugin/PluginManager.php

@@ -90,6 +90,16 @@ class PluginManager
         return $this->plugins;
     }
 
+    /**
+     * Gets global composer or null when main composer is not fully loaded
+     *
+     * @return Composer|null
+     */
+    public function getGlobalComposer()
+    {
+        return $this->globalComposer;
+    }
+
     /**
      * Register a plugin package, activate it etc.
      *