Elixir: Show warning when a package is defined twice in mix.exs

[jeroenvisser101]

Currently, if you have the following in your mix.exs:

  defp deps do
    [{:ecto, "~> 2.1"},
     {:postgrex, "~> 0.13.0"},
     {:ecto, "~> 2.0"}]
  end

The last definition for ecto is ignored and ~> 2.1 is resolved. It could be useful if multiple definitions of the same dependency (in the same file) exist, to show a warning.

[kelvinst]

I can give it a try if no one else is interested. Will start today, maybe I'll open the PR until wednesday.

[whatyouhide]

@kelvinst please do go ahead :) 💟