title: Namespace not found when adding custom dll packed via NuSpec to ASP.NET Core project
body: <p>I have a brand-new asp.net core web api project targeting .net framework 4.6. For business reasons, we need to access an old .net 3.5 library. I ran </p>
  
  <pre><code>nuget spec old-library.dll
  </code></pre>
  
  <p>then </p>
  
  <pre><code>nuget pack old-library.dll.
  </code></pre>
  
  <p>and installed the package. However, I can't seem to access the library.</p>
  
  <pre><code>using old-library-namespace;
  </code></pre>
  
  <p>just doesn't resolve. I know we should be able to use existing .net libraries, and it seems like other people have tried this, but I am at a loss as to the next step here.</p>