Use a proper file extension for the packages file

We'll keep the .txt file for a month or so before deleting it.
Set ERL_LIBS to $(DEPS_DIR) rather than $(DEPS)

This fixes a problem introduced by commit 1b6aa025 such that the
dependencies were no longer being included in compilation.

Noticed due to modules in a project being unable to find behaviours
defined in dependencies.
Merge branch 'fix_lib_dirs' of git://github.com/danielwhite/erlang.mk