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/birdoids/mod.rs

@@ -322,7 +322,7 @@ fn separation_force(boid: Vec2, target: Vec2) -> Option<Force> {
 mod tests {
     use bevy::prelude::*;
 
-    use crate::birdoids_plugin::{cohesive_force, separation_force};
+    use crate::birdoids::{cohesive_force, separation_force};
 
     use super::{Force, BOID_VIEW_RANGE};
 

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,
 };
 

src/main.rs

@@ -1,9 +1,9 @@
 use bevy::prelude::*;
 
-mod birdoids_plugin;
+mod birdoids;
 mod debug_plugin;
 
-use birdoids_plugin::BoidsPlugin;
+use birdoids::BoidsPlugin;
 use debug_plugin::BoidsDebugPlugin;
 
 fn main() {