Should we create some additional concept between api / spi and impl? Something like friend?

How would that look like? In the end it is not a big issue. In the worst case it means that we need to export an impl package (in OSGi metadata) to make it available in another Search bundle.

The problem really exists between engine and orm module. For me it just shows that there split between these two modules is just not as clean and well defined as it should be. I think a few things are easily fixed. Others would require more work and might not be worse the effort atm. I'll update the issue with the concrete classes/packages which cause trouble.

The bigger issue is HSEARCH-1560. Split packages are really bad in OSGi.