Add additional pkgs to test in DrDr.
Split DrDr2 from the main repository.

The `drdr2` pkg is now at
  https://github.com/racket/drdr2
Split drdr from the main repository.

The `drdr` pkg is available at:
  https://github.com/racket/drdr

The pkgs.rktd file is kept to manage the pkgs built
by DrDr on each push.
Split `pkg-build` from the main repository.

The `drdr2` pkg is now at
  https://github.com/racket/pkg-build