[modules] Do basic linking.

test/mjsunit/modules-imports4.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
+
+import {b} from "modules-SKIP-2.js";
+import {a, set_a} from "modules-SKIP-1.js";
+import c from "modules-SKIP-2.js";
+
+assertEquals(1, a);
+assertEquals(1, b);
+assertEquals(42, c);
+
+set_a(2);
+assertEquals(2, a);
+assertEquals(2, b);
+assertEquals(42, c);
+
+assertThrows(() => a = 3, TypeError);
+assertThrows(() => b = 3, TypeError);
+assertThrows(() => c = 3, TypeError);
+
+assertEquals(2, a);
+assertEquals(2, b);
+assertEquals(42, c);