Fix searching for plugins

If a plugin is in a dependency, rebar didn't search for it carefully
enough.

This has a pretty good impact: you no longer need to hardcore plugin directories in most cases. Avoid things like [this](https://github.com/hyperthunk/rebar_dist_plugin/blob/master/rebar.config#L13) and [this](http://lists.basho.com/pipermail/rebar_lists.basho.com/2011-March/000704.html).

In general, I think this should fix most of the plugin loading issues, especially ones that are listed in the dependencies.