All the packages found in 'where' that pass the 'include' filter, but
        not the 'exclude' filter.

def _find_packages_iter(cls, where, exclude, include):
        """
        All the packages found in 'where' that pass the 'include' filter, but
        not the 'exclude' filter.
        """
        for root, dirs, files in os.walk(where, followlinks=True):
            # Copy dirs to iterate over it, then empty dirs.
            all_dirs = dirs[:]
            dirs[:] = []

            for dir in all_dirs:
                full_path = os.path.join(root, dir)
                rel_path = os.path.relpath(full_path, where)
                package = rel_path.replace(os.path.sep, '.')

                # Skip directory trees that are not valid packages
                if ('.' in dir or not cls._looks_like_package(full_path)):
                    continue

                # Should this package be included?
                if include(package) and not exclude(package):
                    yield package

                # Keep searching subdirectories, as there may be more packages
                # down there, even if the parent was excluded.
                dirs.append(dir)