Allow project reference DLLs to be added to the parent nupkg for pack target. It does take regard of sub folders that is relevant especially for satellite assemblies.

Directory.Build.targets

<Project>
  <PropertyGroup>
    <TargetsForTfmSpecificBuildOutput>$(TargetsForTfmSpecificBuildOutput);CopyProjectReferencesToPackage</TargetsForTfmSpecificBuildOutput>
  </PropertyGroup>
  
  <Target Name="CopyProjectReferencesToPackage" DependsOnTargets="BuildOnlySettings;ResolveReferences">
    <ItemGroup>
      <!-- Filter out unnecessary files -->
      <_ReferenceCopyLocalPaths Include="@(ReferenceCopyLocalPaths->WithMetadataValue('ReferenceSourceTarget', 'ProjectReference')->WithMetadataValue('PrivateAssets', 'All'))"/>
    </ItemGroup>

    <!-- Print batches for debug purposes -->
    <Message Text="Batch for .nupkg: ReferenceCopyLocalPaths = @(_ReferenceCopyLocalPaths), ReferenceCopyLocalPaths.DestinationSubDirectory = %(_ReferenceCopyLocalPaths.DestinationSubDirectory) Filename = %(_ReferenceCopyLocalPaths.Filename) Extension = %(_ReferenceCopyLocalPaths.Extension)" Importance="High" Condition="'@(_ReferenceCopyLocalPaths)' != ''" />

    <ItemGroup>
      <!-- Add file to package with consideration of sub folder. If empty, the root folder is chosen. -->
      <BuildOutputInPackage Include="@(_ReferenceCopyLocalPaths)" TargetPath="%(_ReferenceCopyLocalPaths.DestinationSubDirectory)"/>
    </ItemGroup>
  </Target>
</Project>