Rework VirtualPathViewFactory usage of IFileSystem

[loudej]

That's a pretty bad downcast. We should look at how the services dependencies are, and why a file system root path needs to be passed to the compilation service

[pranavkm]

I'm updating that type to use IApplicationEnvironment as part of #54. Missed removing the downcast as part of the first iteration but that would go away.

[pranavkm]

Also, we use the app relative path to generate file names today. I'm not entirely sure it's super important to maintain parity with the current Mvc behavior with this.