apple: Nest all new Apple sources in an apple/ module

We will eventually add `libpthread`, so accommodate for that.

Author: tgross35

src/new/apple_libc/signal.rs src/new/apple/libc/signal.rssrc/new/apple_libc/signal.rs renamed to src/new/apple/libc/signal.rs

@@ -0,0 +1,17 @@
+//! Apple interfaces.
+//!
+//! The Xcode SDK includes interfaces that are split across a couple of different libraries. Most
+//! of these are available at <https://github.com/apple-oss-distributions>.
+
+/// Entrypoint for Apple headers, usually found as part of the Xcode SDK.
+///
+/// <https://github.com/apple-oss-distributions/Libc/tree/main/include>
+mod libc {
+    pub(crate) mod signal;
+    pub(crate) mod unistd;
+}
+
+mod xnu;
+
+pub(crate) use libc::*;
+pub(crate) use xnu::*;