Avoid path when importing modules (#3755)

Rust tooling generally works better this way. This includes
rust-analyzer, but more noticeably the output from `tracing` typically
prints the module path, which did not correspond to the actual file
system before.

Concretely, tracing output from the macOS backend changes from printing:
`winit::platform_impl::platform::util`
To printing:
`winit::platform_impl::macos::util`

src/platform_impl/mod.rs

@@ -1,27 +1,35 @@
 use crate::monitor::{MonitorHandle as RootMonitorHandle, VideoModeHandle as RootVideoModeHandle};
 use crate::window::Fullscreen as RootFullscreen;
 
-#[cfg(windows_platform)]
-#[path = "windows/mod.rs"]
-mod platform;
+#[cfg(android_platform)]
+mod android;
+#[cfg(ios_platform)]
+mod ios;
 #[cfg(any(x11_platform, wayland_platform))]
-#[path = "linux/mod.rs"]
-mod platform;
+mod linux;
 #[cfg(macos_platform)]
-#[path = "macos/mod.rs"]
-mod platform;
+mod macos;
+#[cfg(orbital_platform)]
+mod orbital;
+#[cfg(web_platform)]
+mod web;
+#[cfg(windows_platform)]
+mod windows;
+
 #[cfg(android_platform)]
-#[path = "android/mod.rs"]
-mod platform;
+use android as platform;
 #[cfg(ios_platform)]
-#[path = "ios/mod.rs"]
-mod platform;
-#[cfg(web_platform)]
-#[path = "web/mod.rs"]
-mod platform;
+use ios as platform;
+#[cfg(any(x11_platform, wayland_platform))]
+use linux as platform;
+#[cfg(macos_platform)]
+use macos as platform;
 #[cfg(orbital_platform)]
-#[path = "orbital/mod.rs"]
-mod platform;
+use orbital as platform;
+#[cfg(web_platform)]
+use web as platform;
+#[cfg(windows_platform)]
+use windows as platform;
 
 pub use self::platform::*;
 

src/platform_impl/web/mod.rs

@@ -28,11 +28,9 @@ mod event_loop;
 mod keyboard;
 mod main_thread;
 mod monitor;
+mod web_sys;
 mod window;
 
-#[path = "web_sys/mod.rs"]
-mod backend;
-
 pub use self::device::DeviceId;
 pub use self::error::OsError;
 pub(crate) use self::event_loop::{
@@ -43,6 +41,7 @@ pub use self::monitor::{MonitorHandle, VideoModeHandle};
 pub use self::window::{PlatformSpecificWindowAttributes, Window, WindowId};
 
 pub(crate) use self::keyboard::KeyEventExtra;
+use self::web_sys as backend;
 pub(crate) use crate::icon::NoIcon as PlatformIcon;
 pub(crate) use crate::platform_impl::Fullscreen;
 pub(crate) use cursor::{