Use ILibraryExportProvider as the compilation interface
- Call into the ILibraryExportProvider from the RoslynAssemblyLoader,
  RoslynProjectBuilder and RoslynProjectMetadataProvider
- Simplified caching in RoslynAssemblyLoader and RoslynLibraryExportProvider
- Added a noop IFileWatcher to the host container
- Remove caching and library export functionality from the RoslynCompiler