Reconsider how source-build-reference-packages and source-build-externals are versioned

[MichaelSimons]

These repos have never enabled stable versioning or updated the prerelease moniker. They are always using the "alpha" prerelease label. These are packages are not for "public" consumption, rather they are "internal" flow packages. It smells that you see dependencies on "alpha" versions in the released versions of .NET. These two repos are in a way always in a shippable state. Per a discussion with @mmitche , we both felt it would be better to drop the prelease label or change it to something that better indicates the stability/quality of the package.