Enforce derive cannot be used against serde_core

Even for a simple data structure that would ordinarily expand to an impl
that only refers to the public Serde traits without serde::__private, we
still disallow a (renamed) serde_core dependency. This leaves the
liberty to new usage of private helpers in such impls over time. For
example, similar to the optimization of the standard library's
derive(Debug) that introduced debug_struct_field1_finish to improve
compile time of derived impls.

serde_derive/src/dummy.rs

@@ -22,6 +22,9 @@ pub fn wrap_in_const(serde_path: Option<&syn::Path>, code: TokenStream) -> Token
         )]
         const _: () = {
             #use_serde
+
+            _serde::__require_serde_not_serde_core!();
+
             #code
         };
     }