“Hoppers” might be built with familiar parts, but Pixar’s latest isn’t trying to hide its various influences. Instead, this buoyant, freewheeling adventure about a spirited 19-year-old environmental activist who infiltrates the animal world in the body a robotic beaver wears its references for all to see. Sometimes it’s with a wink; Sometimes it’s more overt, like Kathy Najimy’s flustered scientist shouting, “This is nothing like ‘Avatar!’” Of course, “Hoppers,” directed by Daniel Chong, is a