gpui: Sweep through cargo doc output and mark dubious items as non-public (#3932)

I essentially went through the publicly exported items and marked these
that are e.g. leaky reexports as pub(crate). I expect that'd be done on
Tuesday anyways.

Release Notes:
- N/A

crates/gpui/src/platform.rs

@@ -37,7 +37,7 @@ pub use keystroke::*;
 pub use mac::*;
 #[cfg(any(test, feature = "test-support"))]
 pub use test::*;
-pub use time::UtcOffset;
+use time::UtcOffset;
 
 #[cfg(target_os = "macos")]
 pub(crate) fn current_platform() -> Rc<dyn Platform> {