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/data/themedir1/camelCase/index.php

@@ -2,4 +2,4 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);

tests/phpunit/data/themedir1/sandbox/functions.php

@@ -2,6 +2,6 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);
 
 ?>

tests/phpunit/data/themedir1/sandbox/index.php

@@ -2,6 +2,6 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);
 
 ?>

tests/phpunit/data/themedir1/theme1-dupe/functions.php

@@ -2,6 +2,6 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);
 
 ?>

tests/phpunit/data/themedir1/theme1-dupe/index.php

@@ -2,6 +2,6 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);
 
 ?>

tests/phpunit/data/themedir1/theme1/functions.php

@@ -2,6 +2,6 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);
 
 ?>

tests/phpunit/data/themedir1/theme1/index.php

@@ -2,6 +2,6 @@
 
 // Dummy theme.
 
-echo dirname(__FILE__).'/'.basename(__FILE__);
+echo __DIR__ . '/' . basename(__FILE__);
 
 ?>