Proposal: Refactor packages

[Symbolics]

The current package structure seems to follow some old recommendations in ASDF, the one package per file structure. In practice this has not proven to be a worthwhile structure, except for large project with a great deal of functionality in each file. In Kai, we end up with packages like kai.plotly.launch, containing a single function. In these situations, the overhead of managing a fine grained package structure outweigh the benefits.

This proposal suggest to rationalise Kai under a single namespace, kai and declare it in a top level file.