Fix the implementation of the special Twig loader

[stof]

• throw a Twig_Error_Loader when calling getSource() on a template which is not found. This is the expected behavior of a Twig loader. The previous implementation was triggering a notice for non-matching names and trying to get the content of a non existent file for non-existent paths (which triggers a warning).
• update the exists() implementation to update the internal cache for matched names to avoid a second file_exists check later when getting the source due to the fixed implementation.
• remove the unused dependency on the Publisher.

[everzet]

Thanks Christophe :)