allow arch=all to override No-Support-for-Architecture-all

If a user explicitly requests the download of arch:all apt shouldn't get
in the way and perform its detection dance if arch:all packages are
(also) in arch:any files or not.

This e.g. allows setting arch=all on a source with such a field (or one
which doesn't support all at all, but has the arch:all files like Debian
itself ATM) to get only the arch:all packages from there instead of
behaving like a no-op.

Reported-By: Helmut Grohne on IRC