title: Include source files of package in .NET Core MSBuild
body: <p>In a project.json based project I was able to reference (source) files in a nuget package quite easily. I just added the following to project.json, and I could reference the classes in my code. Even with code-completion.</p>
  
  <pre><code>"dependencies": {
    "Microsoft.Extensions.DotnetToolDispatcher.Sources": {
      "type": "build",
      "version": "1.0.0-*"
    }
  }
  </code></pre>
  
  <p>Now I tried to migrate to the MSBuild based tooling (with tooling version 1.0.0-rc4-004771), but I can't get the same to work in the csproj file.</p>
  
  <p>I've added the following reference to the csproj file:</p>
  
  <pre><code>&lt;ItemGroup&gt;
    &lt;PackageReference Include="Microsoft.Extensions.DotnetToolDispatcher.Sources" Version="1.0.0" PrivateAssets="All" /&gt;
  &lt;/ItemGroup&gt;
  </code></pre>
  
  <p>But after adding the reference, the resources (source files) in the package are still not found. For example, in the project.json project I could simply call the following method, which is present in the package, <code>Microsoft.Extensions.Internal.DotnetToolDispatcher.IsDispatcher()</code>, but the class (or method, etc) is not recognized in my .csproj project.</p>
  
  <p>I think I might have to explicitly add a <code>&lt;Compile Include="x" /&gt;</code> statement, but what do I write in the include? How do I reference the package above?</p>