.NET 8: Source build improvements

[richlander]

The following improvements should be considered for .NET 8.

• .NET 8 Build Plan #2956
• .NET 8: No longer build Windows-specific assets (except on Windows) #2901
• .NET 8: Enable source-build to run from a git repo #2925
• .NET 8: Establish downstream / upstream guidance #2926
• .NET 9: Enable and better define "portable Linux" behavior #2932
• Make call-to-action host errors source-build friendly runtime#71287
• Design for unified behavior controls #2974
• Unified Build Controls #2975
• .NET 8: Repositories dependency info re-work #2979
• .NET 8: Enable VMR/source-build build validation on every PR #2983
• .NET 8: Design E2E testing solution for source-build/VMR #3015
• .NET 8: Tracking for TFM trimming, and elimination of net4* targeting in Linux source-build #3014
• .NET 8: Tooling to identify reference packages used in source-build for a repo #3013
• .NET 8: Tracking issue for repos specifying and regularly updating their dependencies. #2980
• .NET 8: Enable full linux-portable CI build #3033
• .NET 8: Enable experimental MacOS source build #3034

[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.

[mmitche]

@richlander Any objection to me triaging away items here that are definitely not going to be done in .NET 8? E.g. shipping source build as the official build?

[richlander]

Right. Sounds good. Or move to .NET 9.

[mmitche]

Added #2979

[mmitche]

Added #2983

[MichaelSimons]

Closing as .NET 8.0 work has been completed. A few items were pushed to .NET 9.0