Including Unmanaged DLL in NuGet Package Using csproj

I'm developing a .NET Core 2.1 library that depends on an unmanaged DLL. I'd like to include the unmanaged DLL in the NuGet package as well. The problem that I am running into is that if I try to specify all of the information in the .csproj file, the dotnet build process throws the following warning:

warning NU5100: The assembly 'content\lib\subdir\somedll.dll' is not 
    inside the 'lib' folder and hence it won't be added as a reference 
    when the package is installed into a project. Move it into the 
    'lib' folder if it needs to be referenced.

I know that I can embed the unmanaged DLLs by writing a .nuspec (in fact, I have). However, it seems like I shouldn't need to write one with the latest .csproj file format.

Question: How can I use the .csproj file to embed unmanaged DLLs in a NuGet package?

Specifying <ItemGroup><None> in the .csproj file seems to include the files in the output directory but they do not make it into the NuGet package.
Specifying <ItemGroup><Content> in t he .csproj file will get them added to the NuGet package but in the Content directory instead of in the Lib directory.

If I really have to have both a .csproj file and a .nuspec file, what is the best practice for where to put the metadata? In t he .csproj file? In the .nuspec file? Maintain and sync both? Is there a something in the tool chain that can do this for me?

I'm working in Visual Studio Code V1.24, and .NET Core/dotnet V2.1.

Solution

You need to specify explicit package path metadata on the element so that the dll/so/dylib file ends up at the right place in the package so that it is recognised as runtime-specific native DLL:

<ItemGroup>
  <None Include="unmanaged.dll" Pack="true" PackagePath="runtimes\win-x64\native" />
</ItemGroup>