Support dateSkeleton pattern lookup in datagen

[sffc]

Research for https://unicode-org.atlassian.net/browse/CLDR-14993

[sffc]

I need a suggestion involving how to drive @gregtatum's (non-semantic) skeleton API.

Currently I use a components::Bag as the intermediate type. This lets me do things like change the width of the year or set a custom hour cycle when performing skeleton selection.

This has been working fine, but today I discovered that it seems to mess up weekday patterns in a few locales such as fi where there is a difference between c and E. It seems that components::Bag will always look up E and not c.

The type Skeleton supports c vs E, but it has a much more limited API, as essentially only a list of fields.

Do you have a suggestion on how to drive the skeleton selection better?

@zbraniecki