Change serde_path to crate

Also changed the generated code to have at least one thing refer to the
path directly, rather than via `use` -- This shows that the impl *can*
work without `use`, but doesn't actually do all the work to remove the
`use` lines unless we decide we need this feature to work on the 2015
edition

serde_derive/src/de.rs

@@ -60,7 +60,7 @@ pub fn expand_derive_deserialize(input: &syn::DeriveInput) -> Result<TokenStream
         }
     };
 
-    Ok(dummy::wrap_in_const(cont.attrs.serde_path(), "DESERIALIZE", ident, impl_block))
+    Ok(dummy::wrap_in_const(cont.attrs.custom_serde_path(), "DESERIALIZE", ident, impl_block))
 }
 
 fn precondition(cx: &Ctxt, cont: &Container) {