Add support for rebar powered deps

Sets a newly repaired environment variable that rebar checks
to determine the deps folder, REBAR_DEPS_DIR. This way rebar
can now use the erlang.mk deps folder for its own deps.

This is of course incompatible with all rebar versions before
https://github.com/rebar/rebar/commit/620c4b01c6e59d47e92ea069f8510b8cb482ebae
but compatibility should increase with repositories getting
a newer rebar.