Adopting "private" approach for repository and router layers

[pebeto]

The only offline usable layer must be service. Like most of the Julia packages adopting this "private" approach, an underscore (_) could be added to the function naming convention in repository and router layers to avoid suggesting private functions.

# Actual definition
function fetch(::Type{<:Experiment}, id::Integer)::Optional{Experiment}
    ...
end

# Proposed definition
function _fetch(::Type{<:Experiment}, id::Integer)::Optional{Experiment}
    ...
end

[pebeto]

For the router layer, a setup function will be implemented to avoid defining callable route functions.

[pebeto]

Implemented route functions encapsulation in 81c8676.

[pebeto]

I decided to not define repository functions as private. I'm closing this as solved.