haskell.mkDerivation: Support sourceRoot, setSourceRoot

Allow setting `sourceRoot` and `setSourceRoot` in the arguments to
`haskellPackages.mkDerivation`. These arguments are passed directly to
`stdenv.mkDerivation`.

It may be worth reading over `pkgs/stdenv/generic/setup.sh` and checking
if there's other unsupported variables which may be useful. It may also
be worth adding an argument like `extraAttrs` to be passed to
`stdenv.mkDerivation` directly, as an escape hatch.

doc/languages-frameworks/haskell.section.md

@@ -191,6 +191,10 @@ and `version` from Hackage.
 `sha256`
 : Hash to use for the default case of `src`.
 
+`sourceRoot`, `setSourceRoot`
+: Passed to `stdenv.mkDerivation`; see [“Variables controlling the unpack
+phase”](#variables-controlling-the-unpack-phase).
+
 `revision`
 : Revision number of the updated cabal file to fetch from Hackage.
 If `null` (which is the default value), the one included in `src` is used.