Make the analyzer definition provider interfaces API instead of SPI

While they effectively require users to "implement a service", my point is these interfaces are used when building applications, and as such they should be considered API, with every constraint in term of backward compatibility this involves. That's what we did with the `SearchMapping` class, which is used in a very similar way.

Right now this doesn't change anything, since these are experimental interfaces anyway, but if we have to move the interfaces to another package, then the sooner the better.