port extern-prelude test to aux-crate

this still doesn't have the definitive answer, but at least is a working start
.

Author: qmx

src/test/run-make-fulldeps/use-suggestions-rust-2018/Makefile

@@ -12,6 +12,6 @@
 
 pub mod foo {
     pub mod bar {
-        pub struct Baz;
+        pub struct Bazz;
     }
 }

src/test/run-make-fulldeps/use-suggestions-rust-2018/ep-nested-lib.rs renamed to src/test/ui/rust-2018/auxiliary/ep-nested-lib.rs

@@ -8,6 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// edition:2018
+// aux-crate:netted=ep-nested-lib.rs
+
 fn main() {
-    let x = Baz{};
+    let _x = Bazz{};
+    //~^ ERROR cannot find struct, variant or union type `Bazz` in this scope
 }

src/test/run-make-fulldeps/use-suggestions-rust-2018/use-suggestions.rs renamed to src/test/ui/rust-2018/use-suggestions-extern-prelude.rs

@@ -0,0 +1,13 @@
+error[E0422]: cannot find struct, variant or union type `Bazz` in this scope
+  --> $DIR/use-suggestions-extern-prelude.rs:15:14
+   |
+LL |     let _x = Bazz{};
+   |              ^^^^ not found in this scope
+help: possible candidate is found in another module, you can import it into scope
+   |
+LL | use netted::foo::bar::Bazz;
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0422`.

src/test/ui/rust-2018/use-suggestions-extern-prelude.stderr

@@ -1573,7 +1573,8 @@ impl<'test> TestCx<'test> {
         rustc.envs(self.props.rustc_env.clone());
 
         for aux_crate in &self.props.aux_crates {
-            rustc.arg(format!("--extern {}={}", aux_crate.key, aux_crate.value));
+            rustc.arg("--extern");
+            rustc.arg(format!("{}={}/lib{}", aux_crate.key, aux_dir.display(), &aux_crate.value.replace(".rs", ".so").replace("-","_")));
         }
 
         self.compose_and_run(