use extern prelude in Resolver

This fixes two bugs:

- completion for paths works again
- we handle extern prelude shadowing more correctly

crates/ra_ide_api/src/completion/complete_path.rs

@@ -186,4 +186,20 @@ mod tests {
             ",
         );
     }
+
+    #[test]
+    fn completes_use_paths_across_crates() {
+        check_reference_completion(
+            "completes_use_paths_across_crates",
+            "
+            //- /main.rs
+            use foo::<|>;
+
+            //- /foo/lib.rs
+            pub mod bar {
+                pub struct S;
+            }
+            ",
+        );
+    }
 }

crates/ra_ide_api/src/completion/snapshots/completion_item__completes_use_paths_across_crates.snap

@@ -0,0 +1,22 @@
+---
+created: "2019-02-11T11:53:02.410665254Z"
+creator: insta@0.6.1
+source: crates/ra_ide_api/src/completion/completion_item.rs
+expression: kind_completions
+---
+[
+    CompletionItem {
+        completion_kind: Reference,
+        label: "bar",
+        kind: Some(
+            Module
+        ),
+        detail: None,
+        documentation: None,
+        lookup: None,
+        insert_text: None,
+        insert_text_format: PlainText,
+        source_range: [9; 9),
+        text_edit: None
+    }
+]