API: Improve handling of interwiki redirects when resolving

When ApiPageSet resolves redirects, it should add the targets of any
interwiki redirects to its list of interwiki titles.

Also, when outputting the list of redirects it may as well include a
'tointerwiki' property to match the 'tofragment' property it already
outputs.

Bug: T85417
Change-Id: Icf1346f52c8efa099b502433de1b18c4caad09df