update makeElmStuff to makeDotElm, create elm-elm.nix listing the elm
code that elm itself embeds, and pre-fetch it so that the elm build
can function offline. also include a versions.dat file, as created
during an impure build of elm. set ELM_HOME so that the elm build can
find these things.
continues #45448
(cherry picked from commit e7d0df84676d51fb0cbfbeb10dd32153489c3565)
Signed-off-by: Domen Kožar <domen@dev.si>