These are not configuration but business logic, similar to the
canonical names that are in NamespaceInfo.php, these must always
exist and cannot be altered or unset.
They were previously unconditionally assigned during all requests
in Setup.php and passed down as "site configuration".
Changes:
* Move them to MessagesEn.php where they can be cached and
processed the same way as other core-provided aliases.
Document and confirm with tests that this is a mergeable
attribute that follows the language chain.
* Remove the duplicated code in a few places that was reading
this variable + Language::getNamespaceAliases(), to instead
just call the latter and move the logic there, centralised,
and tested.
In doing so I noticed that these were applied in an
inconsistent order. Sometimes the config won, sometimes not.
There's no obvious right or wrong way here, but I've chosen
to standardise on the way that Language::getNamespaceIds() did
it, which is that config wins. This because that method seems
to be most widely used of the three (it decides how URLs and
titles are parsed), and thus the one I least want to change
the behaviour of.
* Document that $wgNamespaceAliases may only be used to
define (extra) aliases, it is and never was a way to access
the complete list of aliases.
Bug: T189966
Change-Id: Ibb14181aba8c1b509264ed40523e9ab4000fd71a
75ccdc6147