title: Index.ts is missing from the TypeScript compilation and include will not solve issue
body: <p>When including <code>@fireflysemantics/slice</code> in an Angular project I get the following error message:</p>
  
  <pre class="lang-none prettyprint-override"><code>ERROR in ./node_modules/@fireflysemantics/slice/index.ts
  Module build failed (from ./node_modules/@ngtools/webpack/src/index.js):
  Error: /home/ole/Temp/fstest/node_modules/@fireflysemantics/slice/index.ts is missing from the TypeScript compilation. Please make sure it is in your tsconfig via the 'files' or 'include' property.
  </code></pre>
  
  <p>I tried including <code>index.ts</code> like this in an <code>include</code> block:</p>
  
  <pre><code>  "include": ["./node_modules/@fireflysemantics/slice/index.ts"],  
  </code></pre>
  
  <p>However, I still get the same error. I posted the reproduction of it in this <a href="https://github.com/angular/angular-cli/issues/16386" rel="nofollow noreferrer">Angular bug report</a>.</p>