title: dotnet restore not copying NETStandard.Library and other framework packages
body: <p>I have a PC which is not allowed to connect to any network. I need to work on this PC on a Net Standard project. I worked in this scenario before with Net Framework projects just using the nuget.exe options to save packages to a folder, copying the folder to the isolated PC and using nuget to restore packages from that folder.</p>
  <p>But doing this with a Net Standard project using dotnet restore mechanism fails because there are missing packages like NETStandard.Library.</p>
  <p>The command I used to create the offline package folder is:</p>
  <pre><code>dotnet restore solution.sln --packages offline_cache
  </code></pre>
  <p>And after copying the folder to the isolated machine I use:</p>
  <pre><code>dotnet restore solution.sln --source offline_cache
  </code></pre>