Refactors BuildHost and MSBuildProjectLoader to allow dotnet-watch to populate workspace in-proc based on already evaluated ProjectInstances (loaded via ProjectGraph).

• Factors language specific retrieval of command line arguments out of ProjectFile into separate types: XyzProjectCommandLineProvider.
• Factors construction of ProjectFileInfo based on ProjectInstance into a separate type: ProjectInstanceReader. The reader takes project instance and optionally MSBuild Project and extracts the necessary info from its properties and items. The Project is optional - it is only needed to populate FileGlobs.
• Adds HotReloadMSBuildWorkspace - this is a replacement for dotnet-watch's IncrementalMSBuildWorkspace
• Adds interface IProjectFileInfoProvider that abstract away retrieval of ProjectFileInfos. MSBuildWorkspace uses an implementation that dispatches to OOP while HotReloadMSBuildWorkspace uses impl that calculates the information directly (in-proc) from project instances provided by dotnet-watch.
• Updates BuildHost project to build first-class NuGet package.

dotnet-watch usage: dotnet/sdk#52163