Flatten the package structure

[binste]

Now that Altair no longer supports different Vega-Lite and even Vega versions, I'd be in favour of simplifying the folder structure for the next major release. This would help with

• version control: Now, all files in new v5, v6, ... folders show up as "new" which makes it difficult to compare against older versions
• type hints: See types are referenced from too deep #3335

I've added a new v6 label we can use to tag issues which we can't tackle just yet but which require a major release.