Need to rationalize our MSBuild package strategy

At the moment we are in a bit of a corner with MSBuild packages. The MSBuild team moves on from TFMs much more aggressively than Roslyn. Consider concretely that in VS 17.14 Roslyn is in a position where we must support `net8.0` in our builds but MSBuild has dropped that from their packages.

To account for this we change the MSBuild package we reference per TFM:

https://github.com/dotnet/roslyn/blob/f4e69647dc4320e1aefddd752ab7939e2610d6f3/eng/Directory.Packages.props#L41-L53

This _works_ but complicates our story a lot in the edges and puts us in places where we need to add TFMs into projects, not because we ship the product, but to work around the NuGet restore issues that result from this split.