[modules] Implement namespace imports.

test/mjsunit/modules-namespace2.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.
+//
+// MODULE
+
+// TODO(neis): Symbol.iterator
+assertEquals(
+    ["b", "c", "get_a", "ns2", "set_a", "zzz", Symbol.toStringTag],
+    Reflect.ownKeys(ns));
+// assertEquals(["b", "c", "get_a", "ns2", "set_a", "zzz"], [...ns]);
+
+import * as foo from "modules-skip-1.js";
+assertSame(foo.a, ns.b);
+assertSame(foo.a, ns.c);
+assertSame(foo.get_a, ns.get_a);
+assertSame(foo.set_a, ns.set_a);
+assertEquals(123, ns.zzz);
+
+assertSame(ns, ns.ns2.ns);
+import * as ns from "modules-skip-namespace.js";
+export {ns};