- This is the first step to removing IVT to Razor assemblies from core Roslyn assemblies. Once Razor is able to consume these changes and move to the new APIs we can entirely remove the IVTs from Roslyn assemblies.
- Brought Razor's `IDynamicFileInfoProvider` implementation over to the ExternalAccess.Razor package and built a new concept (`IRazorDocumentContainer`) that allows Razor to pass it just enough information about the document to Roslyn for it to "do the right thing".

dotnet/aspnetcore#19551

- Now `IDynamicFileInfoProvider` is shimmed and the core-logic exists in the Razor repo.
- Added the ExternalAccess.Razor piece to the Roslyn deployment to ensure our `IDynamicFileInfoProvider` wrapper can be found by MEF components. In the case that the Razor workload is not available we allow a `null` Razor dynamic file implementation.
- Updated the ExternalAccess.Razor project to be netstandard2.0 instead of net472 so it vibes better with the Razor repo.

dotnet/aspnetcore#19551

…e DynamicFileInfoProvider not load Razor assemblies in non-Razor scenarios.