Rewrite `BuilderInsertionPoint.findMessageClasses`

[Current implementation](https://github.com/SpineEventEngine/validation/blob/3b90a0b859a3c79607777d17e93698a58660dfac/java/src/main/java/io/spine/validation/java/BuilderInsertionPoint.java#L102) is way too complex, and may have lead to endless loops.

It has to be rewritten, most likely, with PSI.