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/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::{
@@ -49,3 +47,4 @@ pub(crate) use cursor::{
     CustomCursor as PlatformCustomCursor, CustomCursorFuture,
     CustomCursorSource as PlatformCustomCursorSource,
 };
+use web_sys as backend;