Move all `sys::ext` modules to `os` This PR moves all `sys::ext` modules to `os`, centralizing the location of all `os` code and simplifying the dependencies between `os` and `sys`. Because this also removes all uses `cfg_if!` on publicly exported items, where after #81969 there were still a few left, this should properly work around https://github.com/rust-analyzer/rust-analyzer/issues/6038. `@rustbot` label: +T-libs-impl