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.
Built from https://develop.svn.wordpress.org/trunk@47198


git-svn-id: https://core.svn.wordpress.org/trunk@46998 1a063a9b-81f0-0310-95a4-ce76da25c4cd