This avoids using Mix while compiling which simplifies
a number of things and let us do further build improvements
later on.
Elixir is only enabled from within rabbitmq_cli currently.
Eunit is disabled since there are only Elixir tests.
Dialyzer will force-enable Elixir in order to process
Elixir-compiled beam files.
This commit also includes a few changes that are
related:
* The Erlang distribution will now be started for parallel-ct
* Many unnecessary PROJECT_MOD lines have been removed
* `eunit_formatters` has been removed, it provides little value
* The new `maybe_flock` Erlang.mk function is used where possible
* Build test deps when testing rabbitmq_cli (Mix won't do it anymore)
* rabbitmq_ct_helpers now use the early plugins to have Dialyzer
properly set up
2f0595a95a