Auto merge of #84200 - CDirkx:os, r=m-ou-se

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 rust-lang/rust-analyzer#6038.

`@rustbot` label: +T-libs-impl

clippy_utils/src/paths.rs

@@ -101,7 +101,7 @@ pub const PARKING_LOT_RWLOCK_WRITE_GUARD: [&str; 2] = ["parking_lot", "RwLockWri
 pub const PATH_BUF_AS_PATH: [&str; 4] = ["std", "path", "PathBuf", "as_path"];
 pub const PATH_TO_PATH_BUF: [&str; 4] = ["std", "path", "Path", "to_path_buf"];
 pub const PERMISSIONS: [&str; 3] = ["std", "fs", "Permissions"];
-pub const PERMISSIONS_FROM_MODE: [&str; 7] = ["std", "sys", "unix", "ext", "fs", "PermissionsExt", "from_mode"];
+pub const PERMISSIONS_FROM_MODE: [&str; 7] = ["std", "os", "imp", "unix", "fs", "PermissionsExt", "from_mode"];
 pub const POLL: [&str; 4] = ["core", "task", "poll", "Poll"];
 pub const POLL_PENDING: [&str; 5] = ["core", "task", "poll", "Poll", "Pending"];
 pub const POLL_READY: [&str; 5] = ["core", "task", "poll", "Poll", "Ready"];