title: NuGet package fails to install in .NET Core, but works in .Net Framework. Why?
body: <p>I am working on a Visual Studio solution with two projects which I will call Red.dll and Blue.dll for the purpose of this question. Note that Red.dll depends on Blue.dll. These are both compiled in .NET Standard 2.0 with the output type of "Class Library".</p>
  
  <p>I added the dll of each project to a common NuGet package called "Red" using NuGet Package Explorer. It has the following structure:</p>
  
  <pre><code>lib
  - netstandard2.0
    - Red.dll
    - Red.xml
    - Blue.dll
    - Blue.xml
  </code></pre>
  
  <p>Here is the problem: When I attempt to install this NuGet package to a .NET Core application, I receive the following error:</p>
  
  <p><strong>Error NU1101: Unable to find package Blue.dll. No packages exist with this id in source(s): Desktop, Microsoft Visual Studio Offline Packages, nuget.org</strong></p>
  
  <ol>
  <li>Blue.dll is not a NuGet package. I don't know why it thinks it is.</li>
  <li>Blue.dll is located directly beside Red.dll.</li>
  <li>There are no external dependencies other than Newtonsoft.Json which is irrelevant.</li>
  <li>And to reiterate, Blue.dll is NOT a NuGet package dependency. I realize if you use the "Pack" feature in Visual Studio it will create a dependency, but I created this NuGet package manually to avoid that.</li>
  </ol>
  
  <p><strong>But most importantly</strong>: This all works perfectly in .NET Framework! The NuGet package successfully installed, both dlls are referenced, the code builds without any errors, and the XML documentation is accessible.</p>
  
  <p>Why does this work in .NET Framework but not .Net Core?
  I haven't found anything else online about this particular problem.</p>