reorder crate imports a bit

Author: RalfJung

src/lib.rs

@@ -49,7 +49,10 @@
 // Needed for rustdoc from bootstrap (with `-Znormalize-docs`).
 #![recursion_limit = "256"]
 
-extern crate either; // the one from rustc
+// Some "regular" crates we want to share with rustc
+extern crate either;
+#[macro_use]
+extern crate tracing;
 
 extern crate rustc_apfloat;
 extern crate rustc_ast;
@@ -63,11 +66,8 @@ extern crate rustc_middle;
 extern crate rustc_session;
 extern crate rustc_span;
 extern crate rustc_target;
-#[macro_use]
-extern crate tracing;
-
-// Necessary to pull in object code as the rest of the rustc crates are shipped only as rmeta
-// files.
+// Linking `rustc_driver` pulls in the required  object code as the rest of the rustc crates are
+// shipped only as rmeta files.
 #[allow(unused_extern_crates)]
 extern crate rustc_driver;