Rename & restructure birdoids module

The module isn't the plugin, so it's going to be called simply
"birdoids" going forward.

I've turned it into a folder and a `mod.rs` so I can slap down a small,
custom physics system.

src/debug_plugin.rs

@@ -1,7 +1,7 @@
 use bevy::{prelude::*, window::PrimaryWindow};
 use bevy_spatial::{kdtree::KDTree2, SpatialAccess};
 
-use crate::birdoids_plugin::{
+use crate::birdoids::{
     center_of_boids, velocity_of_boids, Boid, Force, TrackedByKdTree, Velocity,
 };