Pull the trigger on the `pkg/registries` package which acted as a proxy
for `c/image/pkg/sysregistriesv2`. Callers should be using the packages
from c/image directly, if needed at all.
Also make use of libimage's SystemContext() method which returns a copy
of a system context, further reducing the risk of unintentionally
altering global data.
[NO TESTS NEEDED]
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>