Based on TypeScript's [implementation](https://github.com/microsoft/TypeScript/blob/15392346d05045742e653eab5c87538ff2a3c863/src/compiler/binder.ts#L2384-L2393) to correct `SymbolFlags` of `TSModuleDeclaration`.

The `SymbolFlags::NamespaceModule` and `SymbolFlags::ValueModule` have a significant difference,

`NamespaceModule`: can only be referenced as a type.
`ValueModule`: can only be referenced as a value.

Let's take an example to see:

```ts
namespace NamespaceModule {
  export type A = string
}
namespace ValueModule {
  export const A = 0;
}
```

The following code is the JS output of the above example.

```js
"use strict";
var ValueModule;
(function (ValueModule) {
    ValueModule.A = 0;
})(ValueModule || (ValueModule = {}));
```

Only `ValueModule` will be preserved and transformed.

That means whether a `TSModuleDeclaration`  needs to be transformed or removed directly, we can determine by its `SymbolFlags`. We can use it to simplify the current `TSModuleDeclaration` transformation later.