Attributes required by source generators are provided through the Bluehill.Analyzers.Attributes package. To install this package, add the following to csproj:
<ItemGroup>
<PackageReference Include="Bluehill.Analyzers.Attributes" Version="<version>" PrivateAssets="all" ExcludeAssets="runtime" />
</ItemGroup>
There is no reason to include the Bluehill.Analyzers.Attributes assembly in the output directory because the attributes are removed at compile time.