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-mail.php

@@ -8,7 +8,7 @@
  */
 
 /** Make sure that the WordPress bootstrap has run before continuing. */
-require( dirname( __FILE__ ) . '/wp-load.php' );
+require __DIR__ . '/wp-load.php';
 
 /** This filter is documented in wp-admin/options.php */
 if ( ! apply_filters( 'enable_post_by_email_configuration', true ) ) {
@@ -29,7 +29,7 @@ if ( 'mail.example.com' === $mailserver_url || empty( $mailserver_url ) ) {
 do_action( 'wp-mail.php' ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
 
 /** Get the POP3 class with which to access the mailbox. */
-require_once( ABSPATH . WPINC . '/class-pop3.php' );
+require_once ABSPATH . WPINC . '/class-pop3.php';
 
 /** Only check at this interval for new messages. */
 if ( ! defined( 'WP_MAIL_INTERVAL' ) ) {