The IgnoresAccessChecksToAttribute is the reverse of the InternalsVisibleToAttribute - it allows an assembly to declare assemblies whose internals would be visible to it. The attribute class isn't declared in the BCL but is recognized by the CLR (Desktop >= 4.6 and Core), i.e. you can declare it in your code and it would work.
Since there's currently no compiler support for this attribute (I've submitted a PR to Roslyn), this package can be used as a workaround. It generates reference assemblies where all the internal types & members become public, and adds a C# file with the attribute and its instances.
Just add the package and define InternalsAssemblyName items with the assemblies you need access to.
<Project Sdk="Microsoft.NET.Sdk">
<ItemGroup>
<InternalsAssemblyName Include="AssemblyToGrantAccessTo1" />
<InternalsAssemblyName Include="AssemblyToGrantAccessTo2" />
<InternalsAssemblyExcludeTypeName Include="Namespace.TypeName" />
</ItemGroup>
<ItemGroup>
<PackageReference Include="IgnoresAccessChecksToGenerator" Version="0.7.0" PrivateAssets="All" />
</ItemGroup>
</Project>