Prepare falafel balls according to package directions. Sprinkle a pinch of salt on the grated cucumber and allow to sit for a few minutes, then strain out the liquid. To make the tzatziki, combine the cucumber shreds, yogurt, lemon juice, garlic, dill, salt, and pepper and mix well. Gently warm the pita bread, then stuff each with falafel balls, greens, tomatoes, and a generous dollop of tzatziki sauce.