.NET 8: Tooling to identify reference packages used in source-build for a repo

[mmitche]

Describe the Problem

In order to satisfy #2901, we need ways to identify the reference packages currently in use when a repository is built from source, and what code path is pulling those reference packages in.

Describe the Solution

Create tooling to identify the reference packages that are required by a repo source build. The report should include:

• The list of source build reference packages
• How each package is brought in.

Additional Context

This helps drive the TFM reduction.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichaelSimons]

[Triage] @mmitche - we completed the full FX TFM trimming without this. Is there still a need for this tooling?

[mmitche]

Nope! And really, I think you could do this simply by removing the SBRP reference and building.