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/linux/x11/util/icon.rs

@@ -1,7 +1,8 @@
 #![allow(clippy::assertions_on_constants)]
 
+use winit_core::icon::RgbaIcon;
+
 use super::*;
-use crate::icon::RgbaIcon;
 
 pub(crate) const PIXEL_SIZE: usize = mem::size_of::<Pixel>();