Extracting ambient light from light.rs, and creating light directory (#12369)

# Objective
Beginning of refactoring of light.rs in bevy_pbr, as per issue #12349 
Create and move light.rs to its own directory, and extract AmbientLight
struct.

## Solution

- moved light.rs to light/mod.rs
- extracted AmbientLight struct to light/ambient_light.rs

crates/bevy_pbr/src/light/ambient_light.rs

@@ -0,0 +1,39 @@
+use super::*;
+
+/// An ambient light, which lights the entire scene equally.
+///
+/// This resource is inserted by the [`PbrPlugin`] and by default it is set to a low ambient light.
+///
+/// # Examples
+///
+/// Make ambient light slightly brighter:
+///
+/// ```
+/// # use bevy_ecs::system::ResMut;
+/// # use bevy_pbr::AmbientLight;
+/// fn setup_ambient_light(mut ambient_light: ResMut<AmbientLight>) {
+///    ambient_light.brightness = 100.0;
+/// }
+/// ```
+#[derive(Resource, Clone, Debug, ExtractResource, Reflect)]
+#[reflect(Resource)]
+pub struct AmbientLight {
+    pub color: Color,
+    /// A direct scale factor multiplied with `color` before being passed to the shader.
+    pub brightness: f32,
+}
+
+impl Default for AmbientLight {
+    fn default() -> Self {
+        Self {
+            color: Color::WHITE,
+            brightness: 80.0,
+        }
+    }
+}
+impl AmbientLight {
+    pub const NONE: AmbientLight = AmbientLight {
+        color: Color::WHITE,
+        brightness: 0.0,
+    };
+}

crates/bevy_pbr/src/light/mod.rs

@@ -20,6 +20,9 @@ use bevy_utils::tracing::warn;
 
 use crate::*;
 
+mod ambient_light;
+pub use ambient_light::AmbientLight;
+
 /// Constants for operating with the light units: lumens, and lux.
 pub mod light_consts {
     /// Approximations for converting the wattage of lamps to lumens.
@@ -616,45 +619,6 @@ fn calculate_cascade(
         texel_size: cascade_texel_size,
     }
 }
-
-/// An ambient light, which lights the entire scene equally.
-///
-/// This resource is inserted by the [`PbrPlugin`] and by default it is set to a low ambient light.
-///
-/// # Examples
-///
-/// Make ambient light slightly brighter:
-///
-/// ```
-/// # use bevy_ecs::system::ResMut;
-/// # use bevy_pbr::AmbientLight;
-/// fn setup_ambient_light(mut ambient_light: ResMut<AmbientLight>) {
-///    ambient_light.brightness = 100.0;
-/// }
-/// ```
-#[derive(Resource, Clone, Debug, ExtractResource, Reflect)]
-#[reflect(Resource)]
-pub struct AmbientLight {
-    pub color: Color,
-    /// A direct scale factor multiplied with `color` before being passed to the shader.
-    pub brightness: f32,
-}
-
-impl Default for AmbientLight {
-    fn default() -> Self {
-        Self {
-            color: Color::WHITE,
-            brightness: 80.0,
-        }
-    }
-}
-impl AmbientLight {
-    pub const NONE: AmbientLight = AmbientLight {
-        color: Color::WHITE,
-        brightness: 0.0,
-    };
-}
-
 /// Add this component to make a [`Mesh`](bevy_render::mesh::Mesh) not cast shadows.
 #[derive(Component, Reflect, Default)]
 #[reflect(Component, Default)]