feat(engine): support go to definition when function defined in __using__

[katafrakt]

This complements katafrakt/elixir_sense#1 by un-skipping the existing test and adding one more case, which I did not cover initially.

It add working go-to-definition for situations, when a function is defined by a macro in its __using__, for example:

defmodule MyApp.Repo do
  use Ecto.Repo

  def all_but_first(schema) do
    [_ | tail] = al|l(schema)
    tail
  end
end

When editor invokes go-to-definition in the place marked by a pipe, it will go to Ecto.Repo's source file to the line defining the all function.

Similarly, when we have MyApp.Repo.al|l(Thing), it will jump to the same place in the Ecto.Repo's source file.