Figure out what to do with flavors

[davkean]

From @davkean on March 22, 2016 22:18

There's are lots of flavors build on top of csproj/msvbprj including Silverlight, Phone, Store, UWP, Portable and 3rd party flavors. We need to figure out what we are going to do with? Do they port? Or we do we come up a with a way that can they can flavor/extend the base CPS project system.

Copied from original issue: dotnet/roslyn#9982

[davkean]

For those that are unlikely to be ported such as Web Application Projects, we'll want to see if we mimic flavoring similar to what csproj/projectaggregator did. For this to work however, we'll need the ability to override/extend the project node in CPS so that we can add csproj-based interfaces without requiring CPS to implement them directly.