Split physics parts into a submodule

The physics sim bits (that aren't Rapier2d) are now in their own
submodule. I've included the `Wrapping` marker component because it
doesn't really have anywhere else to live, and it's kinda sorta related
to the physics. It controls the motion of objects... that's physics. :p

There are name collisions between Rapier2d's `Velocity` and my own, but
that's a problem for later. I've used more specific type paths where
necessary to maintain the previous behavior.

The `physics::Position` component can go away completely. It's just an
extra copy of some of the built-in `Transform` data. I'm pretty sure it
only exists because I didn't realize I could rely on directly
manipulating the transform when I started this project.

src/ship.rs

@@ -1,7 +1,7 @@
 use bevy::prelude::*;
 use bevy_rapier2d::prelude::*;
 
-use crate::{GameAssets, Position, Rotation, Velocity, Wrapping};
+use crate::{GameAssets, Rotation, physics::Position, physics::Velocity, physics::Wrapping};
 
 #[derive(Component)]
 pub struct Ship;