Tidy up PluginGroupBuilder by moving Plugin index retrieval to it's own function (bevyengine#4446)

# Objective

- Clean up duplicate code in the add_before/add_after functions in PluginGroupBuilder.

## Solution

- moved index retrieval code to a private function index_of() for the PluginGroupBuilder.
- change is just tidying up. No real change to functionality.

Author: its-justus

crates/bevy_app/src/plugin_group.rs

@@ -23,6 +23,22 @@ pub struct PluginGroupBuilder {
 }
 
 impl PluginGroupBuilder {
+    /// Finds the index of a target [`Plugin`]. Panics if the target's [`TypeId`] is not found.
+    fn index_of<Target: Plugin>(&mut self) -> usize {
+        let index = self
+            .order
+            .iter()
+            .position(|&ty| ty == TypeId::of::<Target>());
+
+        match index {
+            Some(i) => i,
+            None => panic!(
+                "Plugin does not exist in group: {}.",
+                std::any::type_name::<Target>()
+            ),
+        }
+    }
+
     /// Appends a [`Plugin`] to the [`PluginGroupBuilder`].
     pub fn add<T: Plugin>(&mut self, plugin: T) -> &mut Self {
         self.order.push(TypeId::of::<T>());
@@ -38,18 +54,7 @@ impl PluginGroupBuilder {
 
     /// Configures a [`Plugin`] to be built before another plugin.
     pub fn add_before<Target: Plugin, T: Plugin>(&mut self, plugin: T) -> &mut Self {
-        let target_index = self
-            .order
-            .iter()
-            .enumerate()
-            .find(|(_i, ty)| **ty == TypeId::of::<Target>())
-            .map(|(i, _)| i)
-            .unwrap_or_else(|| {
-                panic!(
-                    "Plugin does not exist: {}.",
-                    std::any::type_name::<Target>()
-                )
-            });
+        let target_index = self.index_of::<Target>();
         self.order.insert(target_index, TypeId::of::<T>());
         self.plugins.insert(
             TypeId::of::<T>(),
@@ -63,18 +68,7 @@ impl PluginGroupBuilder {
 
     /// Configures a [`Plugin`] to be built after another plugin.
     pub fn add_after<Target: Plugin, T: Plugin>(&mut self, plugin: T) -> &mut Self {
-        let target_index = self
-            .order
-            .iter()
-            .enumerate()
-            .find(|(_i, ty)| **ty == TypeId::of::<Target>())
-            .map(|(i, _)| i)
-            .unwrap_or_else(|| {
-                panic!(
-                    "Plugin does not exist: {}.",
-                    std::any::type_name::<Target>()
-                )
-            });
+        let target_index = self.index_of::<Target>();
         self.order.insert(target_index + 1, TypeId::of::<T>());
         self.plugins.insert(
             TypeId::of::<T>(),