add support for Build-Depends/Conflicts-Arch

add support for Build-Depends/Conflicts-Arch

These new enum values might cause "interesting" behaviour in tools not
expecting them – like an old apt would think a Build-Conflicts-Arch is
some sort of Build-Depends – but that can't reasonably be avoided and
effects only packages using B-D/C-A so if there is any breakage the
tools can easily be adapted.

The APT_PKG_RELEASE number is increased so that libapt users can detect
the availability of these new enum fields via:
 #if APT_PKG_ABI > 500 || (APT_PKG_ABI == 500 && APT_PKG_RELEASE >= 1)

Closes: #837395