[dotnet watch] Replace DotNetWatch.targets with ProjectGraph traversal

[tmat]

Currently we invoke msbuild to evaluate GenerateWatchList target on the root project, which traverses all project dependencies and collects items that are to be watched. The target can be customized by defining CustomCollectWatchItems target. This is currently done by Razor SDK to collect razor and cshtml files.

This approach seems unnecessarily complex as we already create ProjectGraph and traverse it in some scenarios. We could use the evaluation results from the graph to determine watch file list.

The customization is really not very useful as we need to understand what impact a change in each file has on the compilation. We know what the inputs to C#/VB compilation are (compile items and additional items) so we just need to include all of them.

Related: dotnet/aspnetcore#31217