Build Unified Build with n-1 artifacts and sdk

[MichaelSimons]

Doing so will bring the source-build and Microsoft builds closer together and help eliminate differences between the two. It will help support source building the Microsoft build e.g. third parties can offer extended support for .NET. This change means that you can build .NET with a known set of inputs and that you have more assurance that the product can be fully serviced. Building with "arbitrary" inputs makes a system that you cannot always service easily.

Note: With this approach the Microsoft build would require prebuilts e.g. the assemblies outside of the source-build graph.

[dotnet-issue-labeler]

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[dotnet-issue-labeler]

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[mmitche]

I'm not sure this will deliver a desirable result out of the box. Or at least we will need to be careful. It has the possibility of upgrading some dependencies that should probably not be updated, e.g. roslyn could end up depending on newer versions of runtime packages (e.g. System.Text.Json) than it should.

Not saying that this isn't possible. But we have molded the current repository build graph so that dependencies that should not be updated (e.g. roslyn should not depend on the latest runtime). We'll need to be intentional about what gets upgraded and what doesn't, I think.