Remove redundant widget modules in subcrates

Instead, we can define the type aliases just once in the root crate!

graphics/src/layer.rs

@@ -1,12 +1,13 @@
 //! Organize rendering primitives into a flattened list of layers.
 use crate::alignment;
-use crate::image;
-use crate::svg;
 use crate::triangle;
 use crate::{
     Background, Font, Point, Primitive, Rectangle, Size, Vector, Viewport,
 };
 
+use iced_native::image;
+use iced_native::svg;
+
 /// A group of primitives that should be clipped together.
 #[derive(Debug, Clone)]
 pub struct Layer<'a> {