rustc: Eliminate a simple dependency from metadata mod

src/rustc/metadata.rs

@@ -45,8 +45,6 @@ mod back {
 mod driver {
     import session = driver_::session;
     export session;
-    import diagnostic = syntax::diagnostic;
-    export diagnostic;
 }
 
 mod util {

src/rustc/metadata/astencode.rs

@@ -31,7 +31,7 @@ import c = common;
 import e = encoder;
 
 // used in testing:
-import driver::diagnostic;
+import syntax::diagnostic;
 import syntax::codemap;
 import syntax::parse;
 import syntax::print::pprust;