I noticed that the "Packaging Basics" section in the Emacs Lisp manual
suggests that version numbers in `Package-Requires' headers are
optional, however the explanation of `Package-Requires' makes it clear
that the version specifier is necessary and testing confirms this.

The values `foo', `(foo)' and `((foo))' all cause an error to be
signaled by `package-buffer-info'. This seems to me that there is no
"possibly" about it.

It's only a tiny change and I hope it's OK. I have tried to follow the
guidelines found in the Emacs manual about sending patches, I didn't
find any reference about diffs of multiple files, so I hope it's OK to
send the diffs separately.