Move deferred command additions to be processed after `'after_wp_load'` hook.
Move deferred command additions to be processed after `'after_wp_load'` hook.

For the command additions within (mu-)plugins to work, they need to be processed after all plugins have actually been loaded.

Fixes #4122

rodrigoprimo/wp-cli,miya0001/wp-cli,aaemnnosttv/wp-cli,gitlost/wp-cli,GaryJones/wp-cli,aaemnnosttv/wp-cli,trepmal/wp-cli,trepmal/wp-cli,2ndkauboy/wp-cli,2ndkauboy/wp-cli,miya0001/wp-cli,GaryJones/wp-cli,rodrigoprimo/wp-cli,gitlost/wp-cli,gitlost/wp-cli,rodrigoprimo/wp-cli,miya0001/wp-cli,GaryJones/wp-cli,trepmal/wp-cli,aaemnnosttv/wp-cli,2ndkauboy/wp-cli