title: How to import other types in to a typescript .d.ts module
body: <p>I have a custom <code>.d.ts</code> file for an external module that looks like</p>
  <pre><code>declare module 'vertices-bounding-box' {
    export default function boundingBox(positions: [number,number,number][]): [number, number]
  }
  </code></pre>
  <p>Instead of <code>[number,number,number]</code> I would like to use a type from another module like:</p>
  <pre><code>import { vec3 } from 'gl-matrix';
  
  declare module 'vertices-bounding-box' {
    export default function boundingBox(positions: vec3[]): [number, number]
  }
  </code></pre>
  <p>TSC complains when I add the import statement for <code>vec3</code> saying:</p>
  <pre><code>Invalid module name in augmentation. Module 'vertices-bounding-box' resolves to an untyped
  module at '/Users/kevzettler/code/hypeworks/node_modules/vertices-bounding-box/index.js', which cannot be augmented. [2665]
  </code></pre>