Not requiring --use-separate-crates for compiled data

[robertbastian]

Replacing compiled data should not require extra icu4x-datagen flags. This also aligns the config.json more with the CLI arguments (it previously inverted the flag as use_meta_crate to get a true default).

#2945
#3564

[Manishearth]

I'm not opposed to this change but I think I'm missing something about the motivation: "Replacing compiled data should not require extra icu4x-datagen flags". Can you share more details?

We probably should document how to use this; personally I find the non metacrate solution to be more intuitive and less likely to make people try and create cyclic deps.

[robertbastian]

The default datagen mode is using meta crate names, and I want compiled data to work with that. I think that's a reasonable default, because we tell our clients in all our tutorials to use the meta crate.

This change makes individual crates like icu_list usable with compiled data generated using meta crate names. What won't work is using baked output to define a custom DataProvider, this will require using either the meta crate or the --use-separate-crates flag. Using databake output like this is a very niche use case now that we have compiled data.

[Manishearth]

The default datagen mode is using meta crate names

ah, right, forgot about that

+1