title: Force MapStruct to use custom method instead of dto value
body: <p>I have a UserDTO and User entity which I want to map. When creating a new User some fields (for example: <code>password</code>, <code>modifiedBy</code>) must be generated by some custom method (for example: <code>password</code> is randomly generated and encoded, but <code>modifiedBy</code> username is retrieved from security service). For this I autowire some services into the mapper. Many of them return <code>String</code> and MapStruct cannot understand which one to use in each case and just uses the first it found on everything that accepts <code>String</code> as input.</p>
  
  <pre><code>@Mapper(componentModel = "spring", uses = PasswordEncoder.class)
  public interface UserMapper {
  
  @Mapping(target = "password", qualifiedByName = "PASS")
  User mapUser(UserDto dto);
  
  @Named("PASS")
  default String getPass(PasswordEncoder passwordEncoder){
      return passwordEncoder.encode(some_random_generator);
      }
  }
  </code></pre>
  
  <p>This generates code that just uses method from <code>PasswordEncoder</code> in any setter that accepts <code>String</code> and <code>getPass(...)</code> method is not used at all.
  However I need it to use my <code>getPass(...)</code> method on <code>password</code> field only.</p>