Make HelloWorld DATA variable be non-pub

[sffc]

Follow-up from #1947

The data provider APIs should be used to inspect the data. It should not be directly visible. The whole point of HelloWorldProvider is to allow for easy testing of the data provider APIs, and we should use them, not work around them.

CC @robertbastian

[robertbastian]

We need some way to obtain the expected output of the provider. In the fs exporter test I do something like

for each language/value in HelloWorldProvider::DATA
  check that FsDataProvider returns value for language

I don't think we should be using IterableResourceProvider for this, as that's a datagen utility.

[sffc]

I think IterableResourceProvider is the right tool for the job.

I think it doesn't need to hide in the datagen feature?

[robertbastian]

Meanwhile, IterableProvider is and should continue to be a low-level trait with one purpose, to drive the data exporter.

- sffc, #1092 (comment)

That kind of implies gating it on the datagen feature.

[sffc]

OK. Well, #1092 is on my to-do list for the current milestone, and once that is fixed, then maybe we can drive HelloWorldProvider off of that instead of IterableResourceProvider.

[robertbastian]

Fixed by #2152