Update fallback handling with latest MediaWiki core

* Core now no longer does recursive fallbacks, instead each
  language provides its own chain (aside from the universal
  fallback to the translation source, English).

* Update getFallbacks.php script to deal with arrays.
  Also cleaned up the output (using var_export instead of
  manually building PHP, though with post cleanup to keep the
  format somewhat consistent with phpcs and our coding style).

* Added tests to ensure things still work as expected.

* Made most private methods and members protected so that
  subclassing works without shadowing anything.

* Remove redundant @private tags.