This has been inspired by a few commits that cared
for package availability reasons on a particular
architecture; the problem at hand is that pkglists
might need to include groups of packages that are
(un)available on groups of arches, and tackling that
with plain pkg@arch just results in combinatorial
explosion of that matrix.

Arches are handled one-by-one with a few hardcoded
macro substitutions.

Exploding a "pkg@arch1,arch2" string into a set of:

pkg@arch1
pkg@arch2

with subsequent archdep pruning would do the trick;
so here's another sed oneliner that does just that:

$ echo 'pkg@X86,ARM,ppc64le' |
  sed -r ':loop; s/^((([^@]+@)[^,]+)+),([^,]+)/\1\n\3\4/; t loop'
pkg@X86
pkg@ARM
pkg@ppc64le

See-also: 9601a9e7
See-also: 5581dc91
See-also: http://stackoverflow.com/a/55781741/561921