Merge phpBB finder and Symfony finder

The phpBB finder (\phpbb\finder) provides an easy way to search file or classes in extensions and core folders, adding caching and a few other things.
Symfony also provides a finder which have a lot of built-in functionalities that we sometimes need.

It could be nice to merge these 2 finders by making ours extending Symfony's one by providing the extra features we need.

At the same time it could be a good idea to turn this finder into a generator (with a get_all() method to bypass the generator?) to lower the memory consumption (or directly expose the iterator, but it may be more complicated as are currently using one iterator per extension)