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](https://github.com/dotnet/sdk/blob/main/src/BuiltInTools/Watch/HotReload/IncrementalMSBuildWorkspace.cs)
- Adds interface `IProjectFileInfoProvider` that abstract away retrieval
of `ProjectFileInfo`s. `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