generate_index_name() was originally made a utility function in bc65629 because it was shared by both the CreateIndexes (via IndexInput) and Count operations. It was later removed from Count in PHPLIB-285.

Since it is only used for creating indexes, we can consider moving it to a private method within IndexInput. This would also allow us to consolidate tests, which are presently split between the function and IndexInput.

The function has always been documented as @internal so there should be no BC concerns to the move.