Upgrading Microsoft.CodeAnalysis from assembly ref to Nuget package

I'm trying to upgrade a few projects in an old application. I need to upgrade Microsoft.CodeAnalysis.VersionCheckAnalyzer and Microsoft.NetCore.Analyzers from assembly references to Nuget packages. The problem is, I cannot delete the references in Visual Studio. The references don't seem to be in the .csproj file either, nor in the .sln. How can I delete them?