Why are `Plurals` and `PluralRules` separate; they currently output different data

[movermeyer]

There seems to be a lot of overlap between these components. They use the same supplemental/plurals.xml file, but they do so in different ways, so produce different results.

plurals.rb is only output for some locales, but plural_rules.yml gets output for many more (but not all) locales.

My gut feel is that plurals.rb and plural_rules.yml should be output in the same scenarios, and represent the same information.

Figure out how to merge the implementations of these components?