filters: add new getOneImage() helper for manifest/build

This commit adds a helper to find exactly a single image from a
given disto/type/arch description. This will be used in `manifest`,
`build` and potentially more. It is meant to support copy/paste
from the `image-builder list-images` output, so: "distro:centos-9"
is support just like "centos-9" (same for distro/imgType/arch).

cmd/image-builder/export_test.go

@@ -8,6 +8,10 @@ import (
 	"github.com/osbuild/images/pkg/reporegistry"
 )
 
+var (
+	GetOneImage = getOneImage
+)
+
 func MockOsArgs(new []string) (restore func()) {
 	saved := os.Args
 	os.Args = append([]string{"argv0"}, new...)