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.

erlang.mk

@@ -36,6 +36,9 @@ gen_verbose = $(gen_verbose_$(V))
 DEPS_DIR ?= $(CURDIR)/deps
 export DEPS_DIR
 
+REBAR_DEPS_DIR = $(DEPS_DIR)
+export REBAR_DEPS_DIR
+
 ALL_DEPS_DIRS = $(addprefix $(DEPS_DIR)/,$(DEPS))
 ALL_TEST_DEPS_DIRS = $(addprefix $(DEPS_DIR)/,$(TEST_DEPS))