Respect rebar.config.script by using  rebar_config:consult_file/1

Previously when reading deps, the plugin naively read the rebar.config
file using file:consult/1. However, projects can add or remove deps
via a rebar.config.script file. The gproc project uses this mechanism
to provide certains deps only if available.

With this patch, rebar_config:consult_file/1 is used to obtain the
deps data for each project. This will run the script file if present
and give the same result as rebar will see when running `rebar
get-deps`.

Also adding a safety check for missing dependencies to crash with some
useful debug info if this occurs.

Fixes issue #9