fix module shadowing when using CommonJS-like module formatters - fixes #1544

test/core/fixtures/transformation/es6.modules-common/module-shadow/actual.js

@@ -0,0 +1,3 @@
+export function module() {
+  
+}

test/core/fixtures/transformation/es6.modules-common/module-shadow/expected.js

@@ -0,0 +1,8 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.module = _module;
+
+function _module() {}