多文件命名空间

在这里，我们将 Validation 命名空间拆分为多个文件。即使这些文件是分开的，它们也可以贡献给同一个命名空间，并且可以像在一个地方定义它们一样被使用。因为文件之间存在依赖关系，所以我们将添加引用标签来告诉编译器文件之间的关系。我们的测试代码没有改变。

Here, we'll split our Validation namespace across many files. Even though the files are separate, they can each contribute to the same namespace and can be consumed as if they were all defined in one place. Because there are dependencies between files, we'll add reference tags to tell the compiler about the relationships between the files. Our test code is otherwise unchanged.