Add support for rebar powered deps
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.

michaelnisi/erlang.mk,rabbitmq/erlang.mk,dch/erlang.mk,efcasado/erlang.mk,ingwinlu/erlang.mk,a12n/erlang.mk,hairyhum/erlang.mk,yannayl/erlang.mk,KrzysiekJ/erlang.mk,ninenines/erlang.mk,nevar/erlang.mk,jj1bdx/erlang.mk,layerhq/erlang.mk,bsmr-erlang/erlang.mk,crownedgrouse/erlang.mk