Ecto.Repo.Supervisor.compile_config/2 should use ensure_compiled? instead of ensure_loaded?

[evadne]

• Elixir version (elixir -v): 1.7.4
• Ecto version (mix deps): 3.x
• Operating system: macOS

Current behaviour

Currently Ecto.Repo.Supervisor.compile_config/2 uses Code.ensure_loaded?/1 which is fine in most cases, when the adapter lives elsewhere, and would possibly have been compiled already as part of a dependency. But this can be wobbly when both the Repo and the Adapter it refers to are in the same OTP app. When I do this and repeatedly run mix compile on a machine with 8 cores, this fails 50% of the time but never fails during the first compilation pass after running mix clean.

Upon consulting Elixir Documentation on code loading I believe this call should be changed. Once this is changed the wobbliness is no more.