title: Error: Can't resolve '@mat-datetimepicker/core/adapter' - I can't import a module
body: <p>I use the mat-datetimepicker that acts the same as the material datepicker, but add time to it too. I now try to change the format of the displayed datetime. For that I need to overwrite <code>MAT_DATETIME_FORMATS</code>. </p>
  
  <p>My problem comes that I can't import <code>MAT_DATETIME_FORMATS</code>. How I try it:</p>
  
  <pre><code>import { MAT_DATETIME_FORMATS } from '@mat-datetimepicker/core/adapter';
  </code></pre>
  
  <p>And the error I get: </p>
  
  <pre><code>Module not found: Error: Can't resolve '@mat-datetimepicker/core/adapter'
  </code></pre>
  
  <p>If I look at my node_modules folder I find <code>MAT_DATETIME_FORMATS</code> under <code>@mat-datetimepicker/core/adapter</code></p>
  
  <p><a href="https://i.stack.imgur.com/5ojpa.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5ojpa.png" alt="enter image description here"></a></p>
  
  <p>It's in the file datetime-formats.d.ts.</p>
  
  <p>You can also have a look at the repository at hithub:
  <a href="https://github.com/kuhnroyal/mat-datetimepicker/tree/master/projects/core/src/adapter" rel="nofollow noreferrer">https://github.com/kuhnroyal/mat-datetimepicker/tree/master/projects/core/src/adapter</a></p>
  
  <p>But my Import does not work. 
  What do I miss? Why can't it find the module?</p>