Add support for producing intermediate packages from the product source build

[MichaelSimons]

This is needed as part of the Unified Build project. Repo's will no longer have "official builds" (unless they produce assets that ship outside of the core .NET project). The official builds are moving to the VMR. This means the VMR builds need to produce the source build intermediate packages that backflow to the product repos so that they can continue to be used in the repo level source-build legs in PR validation and CI builds.

The production of intermediate packages during product source-builds should be controllable via a flag/property and should be off by default. The intermediates should be an output item only and not actually used within the builds as was the case in the past (8.0 and before). Additionally the intermediates should not be included in the source build artifacts tarball.

[ViktorHofer]

Looks like this should already be possible today by passing in that flag as true: https://github.com/dotnet/arcade/blob/07517cad2726952f1e75d8708cf01ddd089cef04/src/Microsoft.DotNet.Arcade.Sdk/tools/SourceBuild/SourceBuildArcade.targets#L29-L30