$cache->purge() has no effect if $acm_type == dummy

Hi,

One of my extension (and pages extension too) generates dynamic routes.

After create a route, i use $cache->purge() for refresh route collections, but dummy::purge() is empty and url_generator.php is cached, even with dummy cache, so i need to manually clear cache folder.

url_generator.php (or any cached file) should not be cached with dummy cache, or compiled on each page