Only fetch deps compatible for a specific version of Elixir

Questions & Help Questions
4

Plug uses a different lock file to test with older dependencies: plug/mix.exs at main · elixir-plug/plug · GitHub

defmodule Plug.MixProject do
  use Mix.Project

  # ...
  def project do
    [
      # ...
      lockfile: lockfile(),
      # ...
    ]

  defp lockfile() do
    case System.get_env("COWBOY_VERSION") do
      "1" <> _ -> "mix-cowboy1.lock"
      _ -> "mix.lock"
    end
  end
end