Prepare tooling and plan for switch onto the new flat dependency flow

[premun]

Context

As laid out in the Unified Build design documentation, the planned code flow for .NET 9 will change and the individual repositories will only receive and send updates from/to the VMR and not between each other, so the situation looks like this (see VMR Code and Build Workflow for details):

flowchart TD
    subgraph Traditional[Traditional Flow -- .NET 8 and previous]
        tradEmsdk[emsdk]
        tradRuntime[runtime]
        tradWinforms[winforms]
        tradWPF[wpf]
        tradWindowsDesktop[windows-desktop]
        tradEFCore[efcore]
        tradASPNetCore[aspnetcore]
        tradTemplating[templating]
        tradSDK[sdk]
        tradInstaller[installer]
        tradRoslyn[roslyn]
        tradMSBuild[msbuild]
        tradNuGet[nuget]
        tradFSharp[fsharp]

        tradEmsdk-->|Build Outputs|tradRuntime
        tradRuntime-->|Build Outputs|tradWinforms
        tradRuntime-->|Build Outputs|tradTemplating
        tradRuntime-->|Build Outputs|tradSDK
        tradRuntime-->|Build Outputs|tradASPNetCore
        tradRuntime-->|Build Outputs|tradEFCore
        tradWinforms-->|Build Outputs|tradWPF
        tradWPF-->|Build Outputs|tradWindowsDesktop
        tradWindowsDesktop-->|Build Outputs|tradSDK
        tradEFCore-->|Build Outputs|tradASPNetCore
        tradASPNetCore-->|Build Outputs|tradSDK
        tradTemplating-->|Build Outputs|tradSDK
        tradRoslyn-->|Build Outputs|tradSDK
        tradMSBuild-->|Build Outputs|tradSDK
        tradNuGet-->|Build Outputs|tradSDK
        tradFSharp-->|Build Outputs|tradSDK
        tradSDK-->|Build Outputs|tradInstaller
    end

    subgraph VMR[VMR Flow -- .NET 9+]
        vmrRuntime-->vmrVMR
        vmrWinforms-->vmrVMR
        vmrWPF-->vmrVMR
        vmrWindowsDesktop-->vmrVMR
        vmrEFCore-->vmrVMR
        vmrASPNetCore-->vmrVMR
        vmrTemplating-->vmrVMR
        vmrEmsdk-->vmrVMR
        vmrSDK-->vmrVMR
        vmrInstaller-->vmrVMR
        vmrRoslyn-->vmrVMR
        vmrMSBuild-->vmrVMR
        vmrNuGet-->vmrVMR
        vmrFSharp-->vmrVMR

        vmrEmsdk[emsdk]
        vmrRuntime[runtime]
        vmrWinforms[winforms]
        vmrWPF[wpf]
        vmrWindowsDesktop[windows-desktop]
        vmrEFCore[efcore]
        vmrASPNetCore[aspnetcore]
        vmrTemplating[templating]
        vmrSDK[sdk]
        vmrInstaller[installer]
        vmrRoslyn[roslyn]
        vmrMSBuild[msbuild]
        vmrNuGet[nuget]
        vmrFSharp[fsharp]
        vmrVMR[VMR]

        vmrVMR-->vmrEmsdk
        vmrVMR-->vmrRuntime
        vmrVMR-->vmrWinforms
        vmrVMR-->vmrWPF
        vmrVMR-->vmrWindowsDesktop
        vmrVMR-->vmrEFCore
        vmrVMR-->vmrASPNetCore
        vmrVMR-->vmrTemplating
        vmrVMR-->vmrSDK
        vmrVMR-->vmrInstaller
        vmrVMR-->vmrRoslyn
        vmrVMR-->vmrMSBuild
        vmrVMR-->vmrNuGet
        vmrVMR-->vmrFSharp
    end

    Traditional-->VMR

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Goal

We need to assure a smooth transition from the old flow to the new one by preparing a plan for the transition:

• Identify the order in which to move the repositories (bottom/top).
• Schedule communication to repo owners to orchestrate the transition with as few disruptions to developers as possible.
• Create tooling/scripts/manuals for repo owners (or ourselves) to follow that will re-map the current subscriptions onto the VMR.

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