Check for bevy_internal imports in CI (bevyengine#9612)

# Objective

- Avoid using bevy_internal imports in examples. 

## Solution

- Add CI to check for bevy_internal imports like suggested in
bevyengine#9547 (comment)
- Fix another import

I don't know much about CI so I don't know if this is the better
approach, but I think is better than doing a pull request every time I
found this lol, any suggestion is welcome.

---------

Co-authored-by: Rob Parrett <robparrett@gmail.com>

.github/workflows/ci.yml

@@ -333,3 +333,25 @@ jobs:
         with:
           name: msrv
           path: msrv/
+
+  check-bevy-internal-imports:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v3
+      - name: Check for bevy_internal imports
+        shell: bash
+        run: |
+          errors=""
+          for file in $(find examples tests -name '*.rs'); do
+              if grep -q "use bevy_internal" "$file"; then
+                  errors+="ERROR: Detected 'use bevy_internal' in $file\n"
+              fi
+          done
+          if [ -n "$errors" ]; then
+              echo -e "$errors"
+              echo " Avoid importing bevy_internal, it should not be used directly"
+              echo " Fix the issue by replacing 'bevy_internal' with 'bevy'"
+              echo " Example: 'use bevy::sprite::MaterialMesh2dBundle;' instead of 'bevy_internal::sprite::MaterialMesh2dBundle;'"
+              exit 1
+          fi                  

examples/animation/animated_fox.rs

@@ -3,9 +3,7 @@
 use std::f32::consts::PI;
 use std::time::Duration;
 
-use bevy::pbr::CascadeShadowConfigBuilder;
-use bevy::prelude::*;
-use bevy_internal::animation::RepeatAnimation;
+use bevy::{animation::RepeatAnimation, pbr::CascadeShadowConfigBuilder, prelude::*};
 
 fn main() {
     App::new()