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-admin/install-helper.php

@@ -3,7 +3,7 @@
  * Plugins may load this file to gain access to special helper functions for
  * plugin installation. This file is not included by WordPress and it is
  * recommended, to prevent fatal errors, that this file is included using
- * require_once().
+ * require_once.
  *
  * These functions are not optimized for speed, but they should only be used
  * once in a while, so speed shouldn't be a concern. If it is and you are
@@ -35,7 +35,7 @@
  */
 
 /** Load WordPress Bootstrap */
-require_once( dirname( dirname( __FILE__ ) ) . '/wp-load.php' );
+require_once dirname( __DIR__ ) . '/wp-load.php';
 
 if ( ! function_exists( 'maybe_create_table' ) ) :
 	/**