Update plugin structure to self-disable after core-merge

[garretthyder]

Saw this on another feature plugin repo and figured I'd ensure we're meeting this requirement.

Original Post

One requirement for WordPress Core feature plugins is that they need to "self-disable" after they are merged to core.

An easy way to do that is when the plugin's base-file (wp-autoupdates.php) has only the plugin's headers and a check whether the plugin has been merged, and all the functionality/code is in another file that gets included. For example the Lazy Loading Feature Plugin was recently restructured to include a functions.php file for that reason.