ApiBase::getModuleSourceInfo(): Use $wgExtensionDirectory

ApiBase::getModuleSourceInfo(): Use $wgExtensionDirectory

This function takes the class file path, and iterates up through its
parent directories until it hits either a registered extension, or $IP
(for core) or $IP/extensions (for an unknown extension). This change
adds $wgExtensionDirectory as another iteration stopper, so that
unregistered extensions in $wgExtensionDirectory will not cause
iteration back up to the filesystem root, or to $IP if
$wgExtensionDirectory is a child of $IP other than $IP/extensions.

Change-Id: I06553591b0b6156e945aad5176479eb71517e77d