Code Modernization: Replace dirname( __FILE__ ) calls with __DIR__ magic constant.

This avoids the performance overhead of the function call every time `dirname( __FILE__ )` was used instead of `__DIR__`.

This commit also includes:

* Removing unnecessary parentheses from `include`/`require` statements. These are language constructs, not function calls.
* Replacing `include` statements for several files with `require_once`, for consistency:
 * `wp-admin/admin-header.php`
 * `wp-admin/admin-footer.php`
 * `wp-includes/version.php`

Props ayeshrajans, desrosj, valentinbora, jrf, joostdevalk, netweb.
Fixes #48082.

git-svn-id: https://develop.svn.wordpress.org/trunk@47198 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/plugin.php

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