Dataloader.Ecto not compiled with Ecto 2.x

[axelson]

Since commit 8a53e31 changes the optional dependency on ecto to ecto_sql, since my project is still on Ecto 2.x the optional dependency on ecto_sql has no effect. This means that dataloader may be compiled before ecto, which in turns means that the if Code.ensure_loaded?(Ecto) check fails in dataloader_ecto.ex and then my code breaks since it uses Dataloader.Ecto.

I'm not sure what the best course is for this. Some options/thoughts:

1. List a breaking change that only Ecto 3.x is supported
2. Split Dataloader.Ecto into a separate project that has a direct dependency on ecto
   a. This isn't ideal for me but it would've caused the error to appear earlier since I would have noticed ecto_sql being pulled in (or gotten a mix conflict)
3. Revert the dependency change (this should be okay since everything that Dataloader.Ecto uses is actually from ecto and not ecto_sql

[benwilson512]

@axelson yeah I think the only reason we did ecto_sql was for tests I think, can you do a PR where you move it to only: :test ?

[axelson]

@benwilson512 created! #68