fix: ensure the hooked module exports has @@toStringTag property

The README example says the Hook callback `exported` arg is
"effectively `import * as exported from ${url}`".
https://tc39.es/ecma262/#sec-module-namespace-objects specs that
a Module Namespace Object has a @@toStringTag property with value
"Module" and no constructor.

Fixes: nodejs#57

hook.js

@@ -303,7 +303,8 @@ import { register } from '${iitmURL}'
 ${imports.join('\n')}
 
 const namespaces = [${namespaces.join(', ')}]
-const _ = {}
+// Mimic a Module object (https://tc39.es/ecma262/#sec-module-namespace-objects).
+const _ = Object.create(null, { [Symbol.toStringTag]: { value: 'Module' } })
 const set = {}
 
 const primary = namespaces.shift()

test/hook/module-toStringTag.mjs

@@ -0,0 +1,25 @@
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2.0 License.
+//
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2021 Datadog, Inc.
+
+import Hook from '../../index.js'
+import { foo as fooMjs } from '../fixtures/something.mjs'
+import { foo as fooJs } from '../fixtures/something.js'
+import { strictEqual, deepStrictEqual } from 'assert'
+
+let hookedExports
+
+Hook((exports, name) => {
+  hookedExports = exports;
+})
+
+fooMjs
+fooJs
+
+strictEqual(hookedExports[Symbol.toStringTag], 'Module')
+deepStrictEqual(Object.getOwnPropertyDescriptor(hookedExports, Symbol.toStringTag), {
+  value: 'Module',
+  enumerable: false,
+  writable: false,
+  configurable: false
+})