avoid compile-time dependency on modules named in derive

[woylie]

If the @derive attribute is read at compile time, a compile dependency is added to any module that is named in the @derive options. This PR changes the order of the if conditions, so that @derive is only read if the schema has redacted fields. That should avoid unnecessary compile dependencies at least some of the time.

Example:

defmodule Repro.Post do
  use Ecto.Schema
  alias Repro.Other

  @derive {Repro.MyProtocol, some_option: {Other, :something}}
  
  schema "posts" do
    field :name, :string
  end
end

Before the change:

$ mix xref trace lib/post.ex
lib/post.ex:5: alias Repro.Other (runtime)
lib/post.ex:7: require Repro.MyProtocol (export)
lib/post.ex:7: struct Repro.Post (export)
lib/post.ex:7: call Repro.MyProtocol.behaviour_info/1 (runtime)
lib/post.ex:7: call Repro.MyProtocol.Any.__deriving__/3 (compile)
@derive:5: alias Repro.MyProtocol (compile)
@derive:5: alias Repro.Other (compile)

After the change:

$ mix xref trace lib/post.ex
lib/post.ex:5: alias Repro.Other (runtime)
lib/post.ex:7: require Repro.MyProtocol (export)
lib/post.ex:7: struct Repro.Post (export)
lib/post.ex:7: call Repro.MyProtocol.behaviour_info/1 (runtime)
lib/post.ex:7: call Repro.MyProtocol.Any.__deriving__/3 (compile)

This was observed with the ecto_type option and the Flop.Schema protocol: woylie/flop#583.

[josevalim]

Great find! 💚 💙 💜 💛 ❤️