Export rebar3 dependencies for use in nix derivations

Add the plugin to your rebar config:

{plugins, [
    {rebar3nix, {git, "https://github.com/dlesl/rebar3nix.git", {branch, "master"}}}
]}.

Then just call your plugin directly in an existing application:

$ rebar3 nix
===> Fetching rebar3nix
===> Compiling rebar3nix
<Plugin Output>

The plugin will write your dependencies to rebar.nix in a nix-consumable format.