distro: rename awkwardly named function

WithSingleDistro() doesn't follow go's naming convention for creating
objects (New*). Rename it to NewRegistry() and rename the old
NewRegistry() to NewDefaultRegistry().

The idea is that NewRegistry() can be used to create full Registry
objects from outside the package. NewDefaultRegistry() is a convenience
function that creates a Registry with all known distros.

cmd/osbuild-pipeline/main.go

@@ -64,7 +64,7 @@ func main() {
 		}
 	}
 
-	distros := distro.NewRegistry([]string{"."})
+	distros := distro.NewDefaultRegistry([]string{"."})
 	d := distros.GetDistro(distroArg)
 	if d == nil {
 		panic("unknown distro: " + distroArg)