Return argument as a last resort in $(dep_name)

Therefore, if neither $(dep_something) nor $(pkg_something_name) is
defined, $(dep_name) will return the name specified in the various
dependencies lists.

This fixes `core-app-pt-erlc-opts` and `core-deps-fetch-fail-unknown`
because now, the dependency is not "forgotten" anymore. Before,
the dependency directory was missing from $(ALL_DEPS_DIRS) because
$(dep_name) returned nothing.

core/deps.mk

@@ -21,7 +21,7 @@ export DEPS_DIR
 REBAR_DEPS_DIR = $(DEPS_DIR)
 export REBAR_DEPS_DIR
 
-dep_name = $(if $(dep_$(1)),$(1),$(pkg_$(1)_name))
+dep_name = $(if $(dep_$(1)),$(1),$(if $(pkg_$(1)_name),$(pkg_$(1)_name),$(1)))
 dep_repo = $(patsubst git://github.com/%,https://github.com/%, \
 	$(if $(dep_$(1)),$(word 2,$(dep_$(1))),$(pkg_$(1)_repo)))
 dep_commit = $(if $(dep_$(1)_commit),$(dep_$(1)_commit),$(if $(dep_$(1)),$(word 3,$(dep_$(1))),$(pkg_$(1)_commit)))