mw.loader: Create 'skipped' property in registry for debugging

Follows-up 75c08916b02ac8706.

Makes it easier to debug when a module is skipped as right now
they are indistinguishable from regularly loaded modules (other
then checking the network for load.php requests, though those
may not show up either if mw.loader.store is used).

Change-Id: Ic7d2efbf74e0fbf1334f02ecad797e63d0ae2857
Merge "mw.loader: Create 'skipped' property in registry for debugging"