Better handling of missiles splitting to other missiles (#8603)

- Pass a shooter separate to owner into `BulletType#create`. `killShooter` kills the owner while the shooter is passed into the missile ai
- `killShooter` no longer kills already dead units
- `MissileAI` no longer aims at shooter's aim pos if the shooter is dead

core/src/mindustry/ai/types/MissileAI.java

@@ -14,7 +14,7 @@ public class MissileAI extends AIController{
 
         float time = unit instanceof TimedKillc t ? t.time() : 1000000f;
 
-        if(time >= unit.type.homingDelay && shooter != null){
+        if(time >= unit.type.homingDelay && shooter != null && !shooter.dead()){
             unit.lookAt(shooter.aimX, shooter.aimY);
         }