Before I forget ...
The current REQUIRES field is only a bytecompile-time dependency not a
run-time dependency and should *never* be enforced at run-time,
including package fetching.
Since not all run-time dependencies should be enforced either
(eg. Gnus run-time dependency on rmail), the solution isn't a
straightforward download everything.
Suggestions?
New XEmacsen -- 21.2-beta3 and 21.0-pre15-pre6 and package updates
will be available shortly.