title: Namespace added in *.d.ts file in vue typescript is not being recognized without explicit import
body: <p>I am trying to use typescript namespace without importing the <code>.d.ts</code> file in vue typescript project and I have problems.</p>
  <p>I want to use <code>DropdownTypes</code> which is a namespace defined in <code>.d.ts</code> file without importing it.
  I think this should be possible.</p>
  <pre><code>Cannot find namespace 'DropdownTypes'.
  </code></pre>
  <p><code>src/components/Common/Dropdown.vue</code></p>
  <pre><code>&lt;template&gt;
  ...
  &lt;/template&gt;
  
  &lt;script lang=&quot;ts&quot;&gt;
  import { Component, Prop, Vue } from 'vue-property-decorator'
  
  @Component({ name: 'dropdown' })
  export default class Dropdown extends Vue {
    @Prop({ required: true })
    items!: DropdownTypes.DropdownItem[]
  ...
  }
  &lt;/script&gt;
  </code></pre>
  <p><code>src/components/Common/DropdownTypes.d.ts</code></p>
  <pre><code>declare namespace DropdownTypes {
    type DropdownItem {
      id: string;
      text: string;
    }
  }
  </code></pre>
  <p><code>tsconfig.json</code></p>
  <pre><code>{
    &quot;compilerOptions&quot;: {
      &quot;target&quot;: &quot;esnext&quot;,
      &quot;module&quot;: &quot;esnext&quot;,
      &quot;strict&quot;: true,
      &quot;jsx&quot;: &quot;preserve&quot;,
      &quot;allowJs&quot;: true,
      &quot;importHelpers&quot;: true,
      &quot;moduleResolution&quot;: &quot;node&quot;,
      &quot;experimentalDecorators&quot;: true,
      &quot;noUnusedLocals&quot;: true,
      &quot;esModuleInterop&quot;: true,
      &quot;allowSyntheticDefaultImports&quot;: true,
      &quot;sourceMap&quot;: true,
      &quot;noEmit&quot;: true,
      &quot;baseUrl&quot;: &quot;.&quot;,
      &quot;skipLibCheck&quot;: true,
      &quot;types&quot;: [
        &quot;jest&quot;,
        &quot;webpack-env&quot;
      ],
      &quot;paths&quot;: {
        &quot;@/*&quot;: [
          &quot;src/*&quot;,
          &quot;config/*&quot;
        ],
        &quot;vue-analytics&quot;: [
          &quot;src/shims/vue-analytics&quot;
        ],
        &quot;@mathieustan/vue-intercom&quot;: [
          &quot;src/shims/vue-intercom&quot;
        ]
      },
      &quot;lib&quot;: [
        &quot;esnext&quot;,
        &quot;dom&quot;,
        &quot;dom.iterable&quot;,
        &quot;scripthost&quot;
      ]
    },
    &quot;include&quot;: [
      &quot;config/**/*.ts&quot;,
      &quot;src/**/*.ts&quot;,
      &quot;src/**/*.tsx&quot;,
      &quot;src/**/*.vue&quot;,
      &quot;src/**/*.js&quot;,
      &quot;src/**/*.d.ts&quot;,
      &quot;test/**/*.ts&quot;,
      &quot;tests/**/*.ts&quot;,
      &quot;tests/**/*.tsx&quot;
    ],
    &quot;exclude&quot;: [
      &quot;node_modules&quot;,
      &quot;test/e2e&quot;
    ]
  }
  </code></pre>