Remove the "pseudopackage" handling of Architecture: all packages for

Remove the "pseudopackage" handling of Architecture: all packages for
Multi-Arch; instead, Arch: all packages only satisfy dependencies for
the native arch, except where the Arch: all package is declared
Multi-Arch: foreign.  (Closes: #613584)

This has the sideeffect that arch:all packages internally show up as
coming from the native arch - so packages with the architecture "all"
doesn't exist any longer in the pkgcache