cosmetic: move the R-wrapper into its own directory
Move r-packages.nix into the r-modules directory.
R: don't install the "recommended packages"

These packages come with R, but if we install them as part of this build, then
we cannot update them without re-building R as well. Instead, we add those
packages to the R environment through the r-wrapper. This means that
recommended packages can be updated in cran-packgaes.nix, and those updates
have an effect on the installation without re-building R itself.
haskell-defaults: prefer cabal-install 1.20.x on ghc-head