Skip loading PHP shims for JSON i18n files

For now, non-message data (e.g. special page aliases, magic words) belong
in separate files having their own keys in $wgExtensionMessagesFiles. It
is unnecessary to read any PHP files listed under keys that exist in
$wgMessagesDirs; they are merely compatibility shims.

Bug: 63926
Change-Id: I92013d0a45e83ad0f5ac483a3db867806eb71f8a
(cherry picked from commit e0043257c02e866d176fc9cbb7721fa2defcf091)
Update documentation for $wgExtensionMessagesFiles

Follows-up e0043257c02e.

Change-Id: Id8212fca83fde09a89891ead1e0a107ff0d66398
(cherry picked from commit b67db3be8ef5cc9191c2dbdec8809af38596d06d)
Merge "Update documentation for $wgExtensionMessagesFiles" into REL1_23
Merge "Skip loading PHP shims for JSON i18n files" into REL1_23