util: distro: Refactor family and distro code

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

lang/interpret_test/TestAstFunc1/importscope2.txtar

@@ -8,7 +8,7 @@ import "fmt"
 class xclass {
 	import "os" # we can also use a scoped local import
 
-	$aaa = if os.is_debian() { "bbb" }  else { "ccc" }
+	$aaa = if os.is_family_debian() { "bbb" }  else { "ccc" }
 
 	print "${aaa}" {
 		msg => "hello",