chore: import from winit-core instead of the top-level crate

Reproduce with:
find ./src/platform_impl -type f -exec sed -i '' 's/crate::/winit_core::/g' {} \;
find ./src/platform_impl -type f -exec sed -i '' 's/winit_core::platform/crate::platform/g' {} \;
find ./src/platform_impl -type f -exec sed -i '' 's/winit_core::dpi::/dpi::/g' {} \;
cargo +nightly fmt

src/platform_impl/apple/event_handler.rs

@@ -1,7 +1,7 @@
 use std::cell::RefCell;
 use std::{fmt, mem};
 
-use crate::application::ApplicationHandler;
+use winit_core::application::ApplicationHandler;
 
 /// A helper type for storing a reference to `ApplicationHandler`, allowing interior mutable access
 /// to it within the execution of a closure.