erlang: refactor: build packages per Erlang/OTP.

This change introduces a fixpoint, which allows to do deep override when
building packages defined in pkgs/development/beam-modules/default.hex.
This allows to provide beam.packages.erlang{,R16,R17,R18,R19} which
contains the same packages built with different Erlang/OTP versions.

Top-level attribute beamPackages points at beam.packages.erlangR18, the
same applies to other top-level Erlang packages.

TODO:
- beam.packages.erlang{R16,R17} is almost useless, since rebar/rebar3
  does not build using these versions;
- all packages in beam.packages which use buildMix are actually built
  with erlangR18;
- update documentation.