In Object serialization, do not keep track of non-reusable green nodes.

Build NuPkg on Unix

Change our Unix builds to create the set of NuGet packages that are needed for
source build.

Move to dotnet pack

Using the console outputs warning messages which MSBuild interprets as a
build warning and we fail the build due to warn as error being enabled.
Just hide the warnings until we can find a replacement setting.

https://github.com/NuGet/Home/issues/6817

Moved the last of our NuGet package creation over to dotnet. Deleted all
remaining infrastructure for using NuGet.exe.

Moving off of nuget pack to dotnet pack instead. This opens the door for
us to do the packaging on non-Windows machines

Document the use of lazy initialization in TargetFrameworkUtil

Make HostAnalyzerManager a bit lazier

Enable building sourcebuild

Avoid creating all metadata references in the TargetFrameworkUtil static constructor

Avoid IForegroundNotificationService interaction from tests that can't use it

Fixes #25930