Rollup merge of #37198 - jseyfried:future_proof_macros_11, r=nrc

macros 1.1: future proofing and cleanup

This PR
 - uses the macro namespace for custom derives (instead of a dedicated custom derive namespace),
 - relaxes the shadowing rules for `#[macro_use]`-imported custom derives to match the shadowing rules for ordinary `#[macro_use]`-imported macros, and
 - treats custom derive `extern crate`s like empty modules so that we can eventually allow, for example, `extern crate serde_derive; use serde_derive::Serialize;` backwards compatibly.

r? @alexcrichton

src/test/compile-fail-fulldeps/proc-macro/auxiliary/derive-a-2.rs

@@ -1,25 +0,0 @@
-// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// force-host
-// no-prefer-dynamic
-
-#![feature(proc_macro)]
-#![feature(proc_macro_lib)]
-#![crate_type = "proc-macro"]
-
-extern crate proc_macro;
-
-use proc_macro::TokenStream;
-
-#[proc_macro_derive(A)]
-pub fn derive_a(input: TokenStream) -> TokenStream {
-    input
-}

src/test/compile-fail-fulldeps/proc-macro/shadow.rs

@@ -9,13 +9,12 @@
 // except according to those terms.
 
 // aux-build:derive-a.rs
-// aux-build:derive-a-2.rs
 
 #![feature(proc_macro)]
 
 #[macro_use]
 extern crate derive_a;
 #[macro_use]
-extern crate derive_a_2; //~ ERROR: cannot shadow existing derive mode `A`
+extern crate derive_a; //~ ERROR `derive_a` has already been defined
 
 fn main() {}