Fixed fallback behaviour for plural rules

Fixed fallback behaviour for plural rules

Fallbacks didn't work at all for the new plural rule system. I fixed it
by moving the getPluralRules() and getCompiledPluralRules() calls near
to the readPHPFile() calls, before merging is done. Then I factored out
the resulting code to readSourceFilesAndRegisterDeps().

* Removed pluralRules from mergeableMapKeys, it isn't one
* Added compiledPluralRules to allKeys so that it will be merged
* When a language is not present in the CLDR XML files, return null from
  getPluralRules() and getCompiledPluralRules() so that the fallback
  rules won't be overridden with an empty array. Normalised it back to
  an empty array in the unlikely event that there is no plural data in
  the fallback sequence at all, even in English.
* Fixed private function, "protected" is the way to say private here.

Change-Id: I3a008ef7f6ed7aaa15ad25ad796f7a2b8f827fa2