Consolidate the SBRP and SBE repos

[MichaelSimons]

The SBRP and SBE repos have a lot in common today. They both contain packages required only for building .NET from source. The text-only packages in SBRP are not a type of reference package, they contain implementation and are in a type of "external" package. Semantically they should be treated the same as the other source build external packages in SBE.

Benefits of consolidation:

1. Reduced operating costs of maintaining two repos: Merging flow PRs, security work, pipeline maintenance, etc.
2. Improved UX for repo developers - Having one repo to add "source build packages" helps improve the UX. It reduces the concept count and allows repo developers to simply concentrate on the flavors of packages. Note, there can be times when adding a new package requires dependencies across the SBRP and SBE boundary.

Cons:

1. SBRP is an arcade bootstrap repo in source build today. Keeping the amount of code built prior to arcade is a nice to have from a principal perspective.

#5193 is a prerequisite for this work.

Possible names for consolidated repo:

source-build-assets
source-build-packages