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

test/mjsunit/wasm/js-api.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 --allow-natives-syntax
 
 if ((typeof drainJobQueue) != "function") {
   drainJobQueue = () => { %RunMicrotasks() };
 }
 
@@ skipping 21 matching lines @@
 let exportingModuleBinary = (() => {
   var builder = new WasmModuleBuilder();
   builder.addFunction("f", kSig_i_v)
     .addBody([kExprI32Const, 42])
     .exportAs("f");
   return new Int8Array(builder.toBuffer());
 })();
 
 let importingModuleBinary = (() => {
   var builder = new WasmModuleBuilder();
-  builder.addImport("f", kSig_i_v);
+  builder.addImport("", "f", kSig_i_v);
   return new Int8Array(builder.toBuffer());
 })();
 
 // 'WebAssembly' data property on global object
 let wasmDesc = Object.getOwnPropertyDescriptor(this, 'WebAssembly');
 assertEq(typeof wasmDesc.value, "object");
 assertEq(wasmDesc.writable, true);
 assertEq(wasmDesc.enumerable, false);
 assertEq(wasmDesc.configurable, true);
 
@@ skipping 497 matching lines @@
       assertEq(result.instance instanceof Instance, true);
     }
   }
   assertInstantiateSuccess(emptyModule);
   assertInstantiateSuccess(emptyModuleBinary);
   assertInstantiateSuccess(emptyModuleBinary.buffer);
   assertInstantiateSuccess(importingModule, {"":{f:()=>{}}});
   assertInstantiateSuccess(importingModuleBinary, {"":{f:()=>{}}});
   assertInstantiateSuccess(importingModuleBinary.buffer, {"":{f:()=>{}}});
 }