Service classes in wrong package

[emilpaw]

Describe the bug
While working on #311 I noticed that the service classes are placed in the wrong package. Instead of adding them to the service package, they are added to service.impl.

To Reproduce
Generate an application that uses service classes. It doesn't matter if the service classes are created because of the dto or service option.

application {
  config {
    prodDatabaseType postgresql
  }

  entities *
}

entity Category {
  description String required
}
entity Post {
  title String required
}
service Category with serviceClass
dto Post with mapstruct

Expected behavior
The service classes are in the service package and not in impl. All the imports expect the service classes to be in service, and also the base JHipster generator does not use an impl package like that.

[mraible]

@emilpaw Can you please create a PR to fix this?