Check for provided usings

[pascalberger]

Implicit using statements makes working with addins in Cake Frosting much simpler. It would be nice to have an analyzer which would suggest this.

[nils-a]

Currently, we're only doing build-time analyses. So, this might be better placed in the Cake.Addin.Analyzer.

I'll have a look at it. Ideas on how to do this are welcome, though. 😁