title: TypeScript: File 'XXX' is not a module
body: <p>I want to import a module declared in an external file into an other TS file. But VSCode says that the <strong>file 'XXX' is not a module</strong>:</p>
  
  <p><a href="https://i.stack.imgur.com/5q1R3.jpg" rel="nofollow noreferrer">The illustrated error</a></p>
  
  <p>I tried <a href="https://github.com/Microsoft/vscode/issues/10391" rel="nofollow noreferrer">this possible fix</a> but it doesn't work. Any idea?</p>