db/simple: purge special directories for unavailable plugins on update

src/db/plugins/simple/Directory.hxx

@@ -132,6 +132,13 @@ public:
 		return mounted_database != nullptr;
 	}
 
+	/**
+	 * Checks whether this is a "special" directory
+	 * (e.g. #DEVICE_PLAYLIST) and whether the underlying plugin
+	 * is available.
+	 */
+	bool IsPluginAvailable() const noexcept;
+
 	/**
 	 * Remove this #Directory object from its parent and free it.  This
 	 * must not be called with the root Directory.