Refactor: manage .d directories when using sources_list and apt_conf

Refactor: manage .d directories when using sources_list and apt_conf

The .d directories are only managed by the main 'apt' class. However,
both 'sources_list' and 'apt_conf' defines depend on those directories.
So in practice, the defines have an implicit need for those directories
to be somehow managed.

Let's turn this into an explicit relation, and include the directories
in the defines.

This makes it possible to use both defines without having to include the
main 'apt' class. (maybe when using puppet apply?)

Signed-off-by: Gabriel Filion <lelutin@gmail.com>