Update package references instead of emitting duplicates

[kzu]

Right now we're emitting in the targets whichever packages are declared in the source program. We should instead collect them, compare (NuGetVersion-aware) with existing project-declared package references, and emit Include vs Update accordingly.

[kzu]

The built-in nuget tasks already de-duplicate and pick the highest version if it's simultaneousy referenced from the csproj and a file program.