Gives our generated a files an extension that isn't used for any other
purpose (that we know of). This is handy for tooling to be able to
quickly know if a file is 'ours'. This comes up in places like
IVsSymbolicNavigationNotify (go to definition).
* Move path munging in to Razor SDK
* Use AssignTargetPath to determine the target path for outputs and embedded resources
Fixes#1829Fixes#1847Fixes#1999