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

tests/phpunit/tests/filesystem/base.php

@@ -24,7 +24,7 @@ abstract class WP_Filesystem_UnitTestCase extends WP_UnitTestCase {
 		return 'MockFS';
 	}
 	function filter_abstraction_file( $file ) {
-		return dirname( dirname( dirname( __FILE__ ) ) ) . '/includes/mock-fs.php';
+		return dirname( dirname( __DIR__ ) ) . '/includes/mock-fs.php';
 	}
 
 	function test_is_MockFS_sane() {

tests/phpunit/tests/filesystem/findFolder.php

@@ -1,6 +1,6 @@
 <?php
 
-require_once dirname( __FILE__ ) . '/base.php';
+require_once __DIR__ . '/base.php';
 
 /**
  * @group filesystem