Ecto and Phoenix should be optional dependencies

[alexgaribay]

Currently in the dependency tree, both Ecto and Phoenix are dependencies added regardless if the parent project is using either dependency. Ideally, both dependencies should be marked as optional dependencies and have version enforcement if the parent project includes new-relixir as a dependency. The caveat here would be that code that expects Phoenix or Ecto would need to do Code.ensure_loaded? when calling dependency-specific functions. The alternative would be creating additional packages that focus on Phoenix or Ecto respectively.