You can pack them in syrup or water. Doesn't matter because they'll be cooked in the pie eventually. You can use cider, but it's basically just going to be a cloudy concentrated apple juice. You could also start with frozen apple juice concentrate, if you're feeling lazy or want it to be nice and clear.
I don't like the idea of using thickeners in anything that's canned...viscosity and all that. You can always add sugar and flour (or whatever) when you make the pie.