Done by adding isKnownEmpty() to ResourceLoaderModule and overriding it to check for page existence in ResourceLoaderWikiModule. Needed to rearrange some code in OutputPage::makeResourceLoaderLink() to have the emptiness check and dropping of modules work properly. Also factored the page_touched check in ResourceLoaderWikiModule::getModifiedTime() out to a separate method (getTitleMtimes()) and moved in-object caching there as well, so getModifiedTime() and isKnownEmpty() share code and caching for their timestamp/existence checks.
This does not account for the case where e.g. a user has user CSS but no user JS: I had implemented this by checking for $context->getOnly() in getTitleMtimes(), but then realized it's not safe to do this in a function called by getModifiedTime(): it causes the timestamp list in the startup module to only take scripts in account for wiki modules, because the startup module has &only=scripts set
(Almost looks like it could all go into ResourceLoaderModule... But that uses a different version, seemingly, the only one. 3 other subclasses of ResourceLoaderModule implement the same version of getFlip as is moved into a parent class here... Seems daft to have a different version in the base abstract class... Minor oversight?)
Some documentation
TODO:
* Are there instances where we might want to restrict CSS as well as JS?
* Would a $wg config option and/or user preference and/or index.php GET parameter to limit inclusion be useful?
* Can we deprecate any of the existing $wg config options?
* What's going on with the duplicated code between OutputPage and SkinTemplate?
* Interpreted some Trevor-speak in the doc comment of ResourceLoader::preloadModuleInfo().
* Made setMsgBlobMtime() (called from preloadModuleInfo()) actually work, by making getMsgBlobMtime() use the cached blob times if they are available.
* Break long lines.
* Convert long or unnecessary ternary operator usages to if/else.
* Fixed excessively clever assignment expressions.
* Rename $cache to $cacheEntry.
* Removed unnecessary web invocation guards. Their perlish form was making me uncomfortable. BTW, unlike in Perl, die() is not a function, it's a special case in the PHP grammar which very roughly simulates the Perl syntax:
die "x"; // works
0 || die("x"); // works
0 || (die); // works
0 || (die "x"); // fail!
