When a package mentioned in comps is not available in the package set,
print a warning about this. Additionally there is a config option that
allows to turn this warning into a fatal error.
Fixes: https://pagure.io/pungi/issue/50
Fixes: https://pagure.io/pungi/issue/683
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>