inconsistent inclusion of spider engine between centos and ubuntu

If i use the package install guide on mariadb.org then for centos7, the spider engine is installed as part of the yum install. However for ubuntu xenial it is not and must be installed seperately using:
apt install mariadb-plugin-spider

We should be consistent between platforms. I have not tested other platforms but i'd take a guess debian will behave the same as ubuntu.

At least the following files should exist:
/usr/lib/mysql/plugin/ha_spider.so
/usr/share/mysql/install_spider.sql