util: distro: Refactor family and distro code

I hate writing abstraction code like this, but I'm hoping it will be
useful.

examples/lang/pkg1.mcl

@@ -2,5 +2,5 @@ import "os"
 
 pkg "cowsay" {
 	state => "installed",
-	allowunsupported => os.is_debian() ?: true,
+	allowunsupported => os.is_family_debian() ?: true,
 }