chore(swingset): rearrange files: no code changes

Part one of the big reorg: move files around, but don't change any contents.

Main directories now correspond to the bundles that are created and/or the
environments into which they are loaded

* everything in `src/kernel/` goes into the kernel bundle and loaded into the
kernel Compartment
* everything in `src/liveslots/` goes into the liveslots bundle
* `src/supervisors/` contains one directory per worker type, whose contents
get loaded into the corresponding supervisor bundle and loaded into a child
process or Node.js worker thread
* `src/vats/` contains one directory per built-in vat
* `src/devices/` contains one directory per built-in device

All code is free to import from `src/lib/` and from other `@agoric/*` packages within the
agoric-sdk monorepo, under the assumption that they match our general style:
all modules are pure, none import platform builtins, none import third-party
packages (which might violate the other rules).

`src/controller/` builds the controller, which is imported by the host
application into the start compartment.

All imports of platform builtins are under `src/lib-nodejs/` (or, for now,
`src/controller/`), and hopefully this will help us improve ocap discipline
over time by making the primary "build a swingset" API take a batch of powers
instead of importing the platform builtins by itself.

refs #4502