Move plugins to $ARCH/*/mariadb18 to meet multiarch needs (Closes: #739452)

Previously all plugins were installed to usr/lib/mysql/plugin, including
the plugins in the client library package, which inhibited installing
other architecture versions of the same package on the same system,
which is expected to work in multiarch situations.

As client plugins alone cannot change directory with current build options,
and moving the server plugins too does not have any negative effects,
move all plugins to a multiarch directory.

Using the ABI version in the path also makes it possible to install
multiple versions of the shared library side-by-side as is normally
facilitated by the soname .so.18 naming scheme, which the client
plugins did not follow.

See also discussion at https://bugs.debian.org/842454