Split `distro-build` and `remote-shell` pkgs from the main repository.

`distro-build` is at:
  https://github.com/racket/distro-build
`remote-shell` is at:
  https://github.com/racket/remote-shell
Run distro-build and remote-shell on drdr.
Split `plt-web` from the main repository.

Source for `plt-web` is now available at:
  https://github.com/racket/plt-web
Fix test that used `distro-build`.
Fix tests
Drop if preserves marks
Reuse (begin0 x <omitable>)
Drop if preserves marks fix
Move movable
mutable
sclock
(begin x #f) -> (begin x #<void>) (squash)