SPI rework - Preview?


I would like to squash the last two commits before we merge it, but keeping them separate now so you can run the Karaf IT tests and see as well what kind of problems we face in exposing the .impl package.
Therefore this proposal is moving stuff to a new .internal package which is mean to mark that API which we consider private but is used to glue *our* modules together.

 - https://hibernate.atlassian.net/browse/HSEARCH-1739
 - https://hibernate.atlassian.net/browse/HSEARCH-1740