Better shim libraries

I'd love to ditch names like `transformers-compat` and instead allow libraries to declare private dependencies on new versions of themselves.

The harder part of this is private dependencies of course but https://github.com/haskell/cabal/pull/4104 also helps in that by making version bounds that exclude internal components and error today. That way tomorrow we are free to assign brand new meaning to them (i.e. actually depend on a newer version of me, please) tomorrow without breaking any existing packages.