These folders contain, per platform and version,
the bin/mysql_install_db, mysqld, mysql, mysqldump, mysqlcheck, my_print_defaults & share/*

These files come from a standard MariaDB distribution.

This directory used to contain previous versions, which are still available
on Maven Central, but the sources have been removed from the main branch
on the Git repository in #790;
they are still available in Git's history though, of course.