Bootstrap: Use `dirname()` when loading `class-wp-hook.php` from `plugin.php`.

To allow `plugin.php` to be loaded before the rest of WordPress is loaded, it cannot rely on WordPress constants, such as `ABSPATH` and `WPINC`.

Instead, we can assume that `class-wp-hook.php` will be in the same directory as `plugin.php`, so `dirname( __FILE__ )` will give us the correct path to load from.

Props pento, dd32.
Fixes #37707.



git-svn-id: https://develop.svn.wordpress.org/trunk@38589 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/plugin.php

@@ -22,7 +22,7 @@
  */
 
 // Initialize the filter globals.
-require( ABSPATH . WPINC . '/class-wp-hook.php' );
+require( dirname( __FILE__ ) . '/class-wp-hook.php' );
 
 /** @var WP_Hook[] $wp_filter */
 global $wp_filter, $wp_actions, $wp_current_filter;