ww imports: require imports before declarations

test/wcc/data/modfn_coexist_vbu/main.ww

@@ -1,7 +1,7 @@
-// Declaration-before-import twin of ../modfn_coexist/main.ww. The distinct
-// local name follows Go's import/package-scope collision rule while retaining
-// the order-independence check for installing a real import binding.
+// Reordered-source twin of ../modfn_coexist/main.ww. The distinct local name
+// follows Go's import/package-scope collision rule while retaining the
+// declaration/import installation-order check for a real file-scoped binding.
 package main;
-fn localaa() i32 = { return 1; };
 import aa;
+fn localaa() i32 = { return 1; };
 export fn main() i32 = { return localaa() + aa.helper(); };