Create sfxproj concept

Create sfxproj concept

Arcade is more forceful about having a 1-to-1 mapping between project and intermediate path. This made it tougher to get the existing shared framework infra to work that relies on building a "sharedFramework.csproj" once for each shared framework, overriding the intermediate path.

The new approach is to have one sfxproj per shared framework.

Commit migrated from https://github.com/dotnet/core-setup/commit/9fd9b571c590ec32592e5cab3c54b61f9a22d225