webgpu: Refactor webgpu crate (#32255)

* wgpu(_core) -> wgc

* Refactor webgpu crate

split lib.rs into multiple modules

components/script/script_thread.rs

@@ -95,7 +95,7 @@ use style::dom::OpaqueNode;
 use style::thread_state::{self, ThreadState};
 use time::precise_time_ns;
 use url::Position;
-use webgpu::identity::WebGPUMsg;
+use webgpu::WebGPUMsg;
 use webrender_api::units::LayoutPixel;
 use webrender_api::DocumentId;