In Elixir v1.9.0 the Mix.Config was moved from Mix to Elixir's Config module.
A new Config module has been added to Elixir. The previous
configuration API, Mix.Config, was part of the Mix build tool. But since
releases provide runtime configuration and Mix is not included in
releases, we ported the Mix.Config API to Elixir. In other words, use
Mix.Config has been soft-deprecated in favor of import Config.
In Elixir v1.9.0 the Mix.Config was moved from Mix to Elixir's Config module.
See more at: https://elixirforum.com/t/elixir-v1-9-0-rc-0-released/22921