Treat custom derive extern crates like empty modules.

src/librustc_resolve/build_reduced_graph.rs

@@ -250,6 +250,12 @@ impl<'b> Resolver<'b> {
                     self.define(parent, name, TypeNS, (module, sp, vis));
 
                     self.populate_module_if_necessary(module);
+                } else if custom_derive_crate {
+                    // Define an empty module
+                    let def = Def::Mod(self.definitions.local_def_id(item.id));
+                    let module = ModuleS::new(Some(parent), ModuleKind::Def(def, name));
+                    let module = self.arenas.alloc_module(module);
+                    self.define(parent, name, TypeNS, (module, sp, vis));
                 }
             }
 

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() {}