Let's assume we have two projects: ProjectA and ProjectB. ProjectA references ProjectB. When we create a nuget package for ProjectA, by default the package is asking for a nuget package of ProjectB. What if we don't want to create a package for ProjectB, yet we want the referenced files to be included in our nuget package for ProjectA?
Edit the .csproj for Project A and set the following:
<ItemGroup>
<ProjectReference Include="..\ProjectB\ProjectB.csproj">
<PrivateAssets>All</PrivateAssets>
</ProjectReference>
<Content Include="$(OutputPath)\ProjectB.dll">
<Pack>true</Pack>
<PackagePath>lib\$(TargetFramework)</PackagePath>
</Content>
</ItemGroup>
The "PrivateAssets" section means that the package for ProjectB should not ask dependencies. The content/include section shows which files to include in the package - notice, the files are put in the lib folder.