API: Add ApiPageSet accessors for just good and missing titles

ApiPageSet::getAllTitlesByNamespace is fine when that's what you want.
But in all existing uses in core, it's not what's actually wanted.
* ApiQueryBacklinksprop can generate wrong ORDER BY because it considers
  redirect and invalid titles that it doesn't acutally query.
* ApiQueryCategoryInfo raises a fatal error with category redirects.
* ApiQueryDuplicateFiles and ApiQueryImageInfo will add information for
  redirects, leaving them missing the standard data and missing from
  indexpageids.

What all these need is really something along the lines of
"ApiPageSet::getGoodAndMissingTitlesByNamespace". So let's add that,
plus a few similar methods for good measure.

Bug: 71337
Change-Id: I3a205057edcbc303393718e1c8bfb1f1b1f95323
Merge "API: Add ApiPageSet accessors for just good and missing titles"