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
Merge "Skip loading PHP shims for JSON i18n files"