As this was missing, the libmariadb/install.cmake section for Debian
builds was not triggered at all, and existing fixes to make the
Debian paths correct did not apply.

Update libmariadb3 paths in Debian packaging after MariaDB Connector C
3.1.8 upgrade

Also now include caching_sha2_password.so which was previously omitted.

Keep old libmariadbclient.a as symbolic link for backwards compatibility.

Closes: CONC-304
Related: CONC-456, MDEV-22150