This reverts commit 1fc16b4b.

This builds fine locally but not on my Hydra for some reason.

This improves Nix compatibility without changing Stack or Cabal
compatibility. See #728.

This module takes an incredibly long time to compile and I haven't
missed it. If needed, it can be enabled with e.g.
`--arg packages '(p: [ p.ihaskell-widgets ])'`.

Having one source means that everything is rebuilt when e.g an
ihaskell-display package is modified. Splitting the sources out avoids
this by only rebuilding other packages that originate from the same
source.