Stop showing a 'cyclic import' message for unresolved imports

This only happened for single-length import paths, because the import
was being looked up in its own scope.

src/test/compile-fail/bad-module.rs

@@ -1,7 +1,7 @@
 // xfail-stage0
-// error-pattern: nonexistent module
+// error-pattern: unresolved import: vec
 import vec;
 
 fn main() {
-  auto foo = vec.len([]);
+    auto foo = vec::len([]);
 }

src/test/compile-fail/import-loop.rs

@@ -1,7 +1,10 @@
-// error-pattern:cyclic import
+// error-pattern: cyclic import
 
-import x;
+import y::x;
+
+mod y {
+    import x;
+}
 
 fn main() {
-  auto y = x;
 }