Architecture: all + M-A: foreign

[Peter Samuelson <peter@p12n.org>]

In bug #695229, I noted that an Architecture: all package really should
be Multi-Arch: foreign.  This led to an IRC discussion between Goswin,
Steve L. and me in which I formulated the proposal:

    If a package is 'Architecture: all', and all its dependencies are
    'Multi-Arch: foreign' (including the case where there are no
    dependencies), this package should implicitly be treated as
    'Multi-Arch: foreign' as well.

Steve asked what the impact would be, given that Multi-Arch: foreign
only matters if you have reverse dependencies that are not
Architecture: all.  So I wrote a crude script to figure it out.  The
numbers depend on whether you consider only Depends + Pre-Depends, or
if you also consider Recommends and Suggests.

                               Explicit M-A:foreign | Implicit M-A:foreign
                              +---------------------+---------------------
                              |                With |                With
                      | Total |   All  non-arch:all |   All  non-arch:all
Rule variant          |       |          rev-dep(s) |          rev-dep(s)
----------------------+-------+---------------------+---------------------
Depends + Pre-Depends | 17339 |   580           145 |  5310           994
Dep + P-D + Rec + Sug | 17339 |   580           197 |  2969          1151

Now, whether to consider Recommends and Suggests in the calculations, I
don't have a strong opinion.  It does change the mix of packages.  For
example:

- bash-completion could be implicitly M-A:foreign, but this only really
  "matters" if we consider Recommends + Suggests.  Almost nothing
  Depends on it, but several arch:any packages Recommend or Suggest it.

- aptitude-common is implicitly M-A:foreign only if we do _not_
  consider Recommends + Suggests, because while it has no dependencies,
  it Recommends aptitude, which is not M-A:foreign.

- alsa-base is implicitly M-A:foreign only if we do _not_ consider
  Recommends, because while it only Depends on packages which are
  M-A:foreign, it Recommends alsa-utils, which is not.

Anyway, these numbers indicate to me that it may be worth patching
dpkg, apt, aptitude and other deb + multi-arch aware tools, to
implicitly mark all those Arch:all packages as Multi-Arch:foreign, so
that this doesn't have to be done explicitly.

(But not during the freeze.)  Thoughts?