Lazy load entity and repository + presence validation of entity and repo on load

As title
Those `_check*` and `_constantize*` looks redundant both in terms of semantic and checks.
For instance, we double check if the repository is present: `unless repository` in `_check_presence_of_repository` **and** `if repository && ...` in `_constantize_repository!`.

What do you think if we do:

```ruby
def _constantize_repository!
  @repository = Utils::Class.load!(repository)
rescue NameError
  raise "meaningful exception"
end
```