Commit 5fa79061 ("Implement support for repos with modern-style
arch:all support") added support for modern-style arch:all - where the
architecture-specific release files do not list packages for arch all;
they're in a separate list.

However, it did not work for foreign bootstrap when the second stage is
performed separately.  The second stage relied on a global
"ARCH_ALL_SUPPORTED" being set by the first stage, which doesn't happen
when they are executed separately.  This caused PureOS Crimson to fail
a foreign bootstrap, because dpkg could not record the selection of
cron-daemon-common (an arch:all pre-dependency of cron).

Additionally, it appears this would not have been precisely correct for
multiple suites.  Most logic would use the ARCH_ALL_SUPPORTED value of
the last suite for all suites.  download_release_indices would have
carried over the 'all' from prior suites to later suites, possibly
duplicating it if more than one suite supports all.

Don't use global state for this.  Check support for each suite when
needed.

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>