discover apps from deps

[Benoit Chesneau]

Right now it seems that the elixir applications are discovered from a static folder:
https://github.com/barrel-db/rebar3_elixir/blob/master/src/rebar3_elixir_util.erl#L29

Instead we should use applications listed in deps and check if mix.exs is present and then compile it using mix until rebar.config is not present. Thoughts?

[Sushruth Sivaramakrishnan]

Checks for mix.exs, if present uses mix to compile