R: don't recurse into the rPackages set to conserve memory and CPU time

The addition of CRAN has made "nix-env -qa \*" run ~30% longer than before [1].

[1] https://github.com/NixOS/nixpkgs/commit/0d264c17616646a7d68e0a465fdc0f335e0d2fb3
haskell-HTTP: update to version 4000.2.14
haskell-contravariant: update to version 0.5.2
haskell-distributive: update to version 0.4.4
haskell-gtk-traymanager: update to version 0.1.4
haskell-hourglass: update to version 0.1.2
haskell-html-conduit: update to version 1.1.0.5
haskell-http-client: update to version 0.3.2.2
haskell-monad-logger: update to version 0.3.6.1
haskell-optparse-applicative: update to version 0.8.1
haskell-pandoc-citeproc: update to version 0.3.1
haskell-resourcet: update to version 1.1.2.1
haskell-transformers-compat: update to version 0.3
haskell-yesod-core: update to version 1.2.15.1
haskell-syb-0.4.1: disable the test suite when compiling with GHC HEAD

The test suite fails in a seemingly harmless way:

  ### Failure in: All:0
  expected: "(MyDataType Int,(DataType {tycon = \"MyDataType\", datarep = AlgRep [MyDataType]},(\"\",(\"MyDataType\",(\"\",\"MyDataType\")))))"
   but got: "(MyDataType Int,(DataType {tycon = \"Datatype.MyDataType\", datarep = AlgRep [MyDataType]},(\"\",(\"MyDataType\",(\"Datatype\",\"MyDataType\")))))"
haskell-comonad: update to version 4.2
haskell-derive: update to version 2.5.16
haskell-HsSyck: update to version 0.52
haskell-JuicyPixels: update to version 3.1.5.2
haskell-concurrent-extra: update to version 0.7.0.7