All packages that at the moment depend directly on mysql-client should instead have something like:

 Depends: the-one-they-tested-with | virtual-mysql-client

(or Suggests or Recommends)

At the moment in unstable the packages mysql-server-5.5 and mariadb-server-5.5 have

 Provides: mysql-virtual-server

and mysql-client-5-5 and mariadb-client-5.5 have

 Provides: mysql-virtual-client

Later when other versions are uploaded to Debian (e.g. MySQL 5.6, MariaDB 10, Percona etc) they will include the same provides as long as they are compatible enough with MySQL 5.5 to be drop-in-replacements. (Side note: Drizzle is a MySQL fork but is not an drop-in-replacement and thus does not have this provides.)