We already ship libmysqlclient18 that replaces any existing package
with the same name and install symlinks that point to out libmariadb.so.

Instead of creating more packages, we add just one more that does the
same and provides both libmysqlclient19 and -20 replacements symlinks.

This is a clean solution, because systems that only use (and are built
against) libmariadb will not need them and if somebody has done a mistake
in the linker parameters, they will spot it and can easily update the
filename to libmariadb.