Split almost everything else from the main repository.

The source to the split packages is in repositories under the
`racket` organization on GitHub. The repositories are all named
according to the pkg name, except for multiple-package
repositories such as `racket/compiler` which is named based on the
old directory name without the `-pkgs` suffix. Thus

   `pkgs/compiler-pkgs` -> https://github.com/racket/compiler

The Makefile has also been adjusted to pull packages from the
catalog when you type `make`. This currently relies on some tricks
that will break if you try to specify a particular set of `PKGS` on
the command line. We plan to improve this soon.

The packages in `pkgs/racket-pkgs` and `pkgs/base` are staying in
the repository, since they logically belong with the core code.

The `plt-services` package is still in the repository, but will
move out soon.