Organize neo datetime provider code

[Manishearth]

Fixes #5613

I used the same organization as in #7386: names and semantic_skeleta. Glue patterns go in names for now, but they're easy to move.

[sffc]

don't love the churn but ok

[sffc]

The stuff in components/datetime/src/provider/semantic_skeletons.rs is not really about semantic skeleta. It's just a bunch of data marker attributes, for looking up names as well as patterns. I think the file is a misnomer.

Glue patterns definitely don't belong in a file named names.rs. It's about as far as you can get away from names in datetime.

[Manishearth]

Pure rebase over Rob's PR, no changes.

@sffc The glue key is a minor thing that can be moved later, I did this to be consistent with the choice we made in provider/source. We could just have glue.rs in both, in provider/source the code was very closely related so it stayed together.

Happy to rename semantic_skeleton.rs if you have a suggestion. Things can also be further organized.

I have a preference for merging this as is since this splits the module. After a split like this, further renames are much easier to do.

[sffc]

Needs another rebase.

[Manishearth]

Done