Stop treating text-only packages as reference packages

[MichaelSimons]

Currently SBRP has three types of packages: reference, target and text-only. Text-only packages are not reference packages rather they contain implementation. The are in a way a type of "external" package like the source-build-externals but are produced with a different construct.

Text-only packages are not tracked by the poison infrastructure. Reference and target packages are prohibited in the build output while text-only packages are allowed. Because text-only packages are produced from SBRP, they appear in vmr's prereqs/packages/reference folder rather than "regular" packages. Currently there is no reason to include the reference packages in the previous source-built artifacts other than the text only packages.

Text-only packages should appear as standard packages in the build output and no longer treated as reference packages.