Log Message:
bulk-medium/large: rearrange emacs again.

At gdt's suggestion, put emacs-nox11 in bulk-medium; it doesn't pull
in rust. The x11 emacs, which does, goes in -large, as does
emacs-packages, which will pull it in by default.

As before (but this wasn't visible in -medium's commit log) depend on
the meta-package instead of an explicit emacs version.

This moves the version of emacs we're building from 26 to 28. As best
anyone's been able to figure, the only reason these were building 26
was bitrot. :-|

While here,
  - don't list xemacs in both -medium and -large; it goes in -medium;
  - list xemacs along with xemacs-packages; this does not change what
    actually happens, but makes things a lot clearer to passersby.