libextra: Rename the actual metadata names of libcore to libstd and libstd to libextra

src/libsyntax/print/pprust.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::prelude::*;
 
 use abi::AbiSet;
 use ast::{RegionTyParamBound, TraitTyParamBound, required, provided};