Include KeyOutput too in case the other output group is empty

Depending on the type of project, sometimes the BuiltProjectOutputGroupOutput is
empty (i.e. in our tasks it was), but the key output was properly populated.

Add both and ensure no duplicate PackageFiles are created after the inference is
done.

Author: kzu

src/Build/NuGet.Build.Packaging.Tasks/NuGet.Build.Packaging.Inference.targets

@@ -53,7 +53,7 @@ Copyright (c) .NET Foundation. All rights reserved.
 		</PropertyGroup>
 
 		<ItemGroup>
-			<_InferredPackageFile Include="@(BuiltProjectOutputGroupOutput -> '%(FinalOutputPath)')"
+			<_InferredPackageFile Include="@(BuiltProjectOutputGroupOutput -> '%(FinalOutputPath)');@(BuiltProjectOutputGroupKeyOutput -> '%(FinalOutputPath)')"
 							   Condition="'$(IncludeOutputsInPackage)' == 'true' and '$(IsPackagingProject)' != 'true'">
 				<!-- Packaging projects don't contribute primary output -->
 				<Kind>$(PrimaryOutputKind)</Kind>
@@ -154,7 +154,7 @@ Copyright (c) .NET Foundation. All rights reserved.
 				NOTE: if PackageId is empty, we won't generate a manifest and it means the files need to be packed with the
 				current project.
 			-->
-			<PackageFile Include="@(_InferredPackageFile)">
+			<PackageFile Include="@(_InferredPackageFile -> Distinct())">
 				<Source>Implicit</Source>
 				<PackageId Condition="'$(IsPackable)' == 'true'">$(PackageId)</PackageId>
 				<Platform>$(Platform)</Platform>