erlang: put Elixir/LFE into per-OTP packageset.

This makes beam.package.erlangR19.abnf to be actually built with R19,
instead of the default R18. It means that Elixir and LFE are provided in
two versions, one built with R18 and with R19.

Please note that Elixir does not build with R16 and R17 - trying to
access beam.packages.erlang{R16,R17}.elixir will throw an error.