Add some sensible defaults if rebar.config is found

[plux]

Description

If we find a rebar.config file (and no erlang.mk!) we will use rebar3's directory structure as defaults for deps_dirs, apps_dirs and include_dirs.

Note that this is just defaults, so they will be overridden if there is an erlang_ls.config file either locally or globally.

This will hopefully improve the experience for new users when jumping into a rebar3 project.