Revert "Follow up on r43982. Reduce dirname(__FILE__) calls in core and extensions."

Revert "Follow up on r43982. Reduce dirname(__FILE__) calls in core and extensions."

Uses $dir in extension files, and assumes that it remains unchanged in require_once( 'maintenance/commandLine.inc' ).
In fact, it is likely that '$dir' will be set when setting up command-line, as some extensions will use the same var.

Recommended fix: Use $CentralAuth_dir, $EmailPage_dir, etc.