[wasm] Implement correct 2-level namespace for imports.

test/mjsunit/wasm/gc-buffer.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-wasm  --stress-gc --expose-gc
 
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
 function run(f) {
   // wrap the creation in a closure so that the only thing returned is
   // the module (i.e. the underlying array buffer of WASM wire bytes dies).
   var module = (() => {
     var builder = new WasmModuleBuilder();
-    builder.addImport("the_name_of_my_import", kSig_i_i);
+    builder.addImport("mod", "the_name_of_my_import", kSig_i_i);
     builder.addFunction("main", kSig_i_i)
       .addBody([
         kExprGetLocal, 0,
         kExprCallFunction, 0])
       .exportAs("main");
     print("module");
     return new WebAssembly.Module(builder.toBuffer());
   })();
 
   gc();
   for (var i = 0; i < 10; i++) {
     print("  instance " + i);
-    var instance = new WebAssembly.Instance(module, {the_name_of_my_import: f});
+    var instance = new WebAssembly.Instance(module, {"mod": {the_name_of_my_import: f}});
     var g = instance.exports.main;
     assertEquals("function", typeof g);
     for (var j = 0; j < 10; j++) {
       assertEquals(f(j), g(j));
     }
   }
 }
 
 (function test() {
   for (var i = 0; i < 3; i++) {
     run(x => (x + 19));
     run(x => (x - 18));
   }
 })();