ext.mantle.modules needs ext.mantle

Otherwise it silently fails to extend an undefined mw.mantle, and
ext.mantle.oo gets TypeError: M is undefined, because mw.mantle is
undefined.

Tested by separate mw.loader.load() of ext.mantle, ext.mantle.oo,
ext.mantle.modules, ext.mantle.templates in a fresh page that doesn't
use Mantle.

Change-Id: I5fbade37719c6faf5f6a31f0d48c012dbcf2c91a
Merge "ext.mantle.modules needs ext.mantle"