Move dependencies callbacks back into Mix.Deps.Converger

The converger is the one that knows if a dependency exists or
not and, as such, is the best one to know if a dependency callback
should be invoked or not.

This commit also normalizes some of the Retriever internal
functions, for example, now we have mix_children and rebar_children.
The goal is to extract those functions (and the ones in Mix.Deps)
one by one into a Mix.DepsManager behaviour which will be implemented
by Mix.DepsManager.Mix and Mix.DepsManager.Rebar, removing all
the conditional calls existing in the code.
Correctly add load_paths for rebar dependencies

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>