Force disable warnings_as_errors for dependencies

[ericmj]

Warnings as errors causes more harm than good when it's enabled for dependencies. We should disable it for both the elixir and erlang compilers.

For makefiles we already added the IS_DEP=1 env var to disable it for erlang.mk. I will investigate how to do it for rebar3, I've got some ideas from the rebar3 team already. With mix we, of course, have full control over the options.

[josevalim]

I have fixed this for mix too. So just rebar/rebar3 left. :)

[josevalim]

@ericmj and i am almost sure you can do it for rebar3 by using overrides?

[josevalim]

This is pending a new Rebar 3 release.

[ericmj]

Reopening to remind me of updating mix local.rebar.