Created tests/rustdoc/macro subfolder to limit number of files at the top level

Author: GuillaumeGomez

tests/rustdoc/auxiliary/external-macro-src.rs renamed to tests/rustdoc/macro/auxiliary/external-macro-src.rs

@@ -0,0 +1,20 @@
+pub struct Option;
+impl Option {
+    pub fn unwrap(self) {}
+}
+
+mod macros {
+    use crate::Option;
+    /// [`Option::unwrap`]
+    #[macro_export]
+    macro_rules! print {
+        () => ()
+    }
+}
+
+mod structs {
+    use crate::Option;
+    /// [`Option::unwrap`]
+    pub struct Print;
+}
+pub use structs::Print;