Alloy CEO Leslie Morgenstein, TV chief Gina Girolamo and film leader Elysa Koplovitz Dutton, leaders of the company behind such TV and film hits as “Gossip Girl,” “Pretty Little Liars,” “You,” “Vampire Diaries” and “You Are So Not Invited to My Bat Mitzvah” explain how they’ve thrived in the YA space and why they are now looking to grow up a bit.