There are several plugins in ha_spider: spider, spider_alloc_mem,
spider_wrapper_protocols, spider_rewrite etc.

INSTALL PLUGIN foo SONAME ha_spider causes all the other ones to be
installed by the init queries where foo is any of the plugins.

This introduces unnecessary complexiy. For example it reads
mysql.plugins to find all other plugins, causing the hack of moving
spider plugin init to a separate thread.

To install all spider related plugins, install soname ha_spider should
be used instead.

This also fixes spurious rows in mysql.plugin when installing say only
the spider plugin with `plugin-load-add=SPIDER=ha_spider.so`:

select * from mysql.plugin;
name	dl
spider_alloc_mem	ha_spider.so # should not be here
spider_wrapper_protocols	ha_spider.so # should not be here

Adapted from part of the reverted commit
c160a115.