Remove extern crate rustc_middle from rustc_const_eval.

This requires exporting the interpreter macros so they can be used with
`use crate::interpret::*`.

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -1,5 +1,6 @@
 // Not in interpret to make sure we do not use private implementation details
 
+use rustc_middle::bug;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::InterpErrorInfo;
 use rustc_middle::query::{Key, TyCtxtAt};