Find listing deps on first run

fix #1032

This fixes the issues with:
.  render_posts:cache/stories/listings-demo.html

but not with:
.  render_pages:output/stories/listings-demo.html

It would be easy to add the same fix render_pages. But I think render_pages should depend only to the generated post fragment. So the fix would be to never include the '.dep' contents into the render_pages tasks. I didnt double check this so didnt implement it.