Since Debian 9 (Stretch) and Ubuntu 16.10 (Yakkety) the following
packages have existed:
* virtual-mysql-client
* virtual-mysql-client-core
* virtual-mysql-server
* virtual-mysql-server-core
* virtual-libmysqlclient-dev

They are metapackages that in Debian depend on MariaDB and in
Ubuntu currently on MySQL. We need to provide them and point
them to MariaDB so that systems that have the mariadb.org
repositories enabled automatically get everything MariaDB
and not MySQL.

This change makes the packaging provide the four first ones,
and later commits will fix the client library issues.