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/importscope0.txtar

@@ -9,7 +9,7 @@ import "fmt"
 class xclass {
 	#import "os" # this should not be required, top-level should be enough
 
-	$aaa = if os.is_debian() { "bbb" }  else { "ccc" }
+	$aaa = if os.is_family_debian() { "bbb" }  else { "ccc" }
 
 	print "${aaa}" {
 		msg => "hello",

lang/interpret_test/TestAstFunc1/importscope1.txtar

@@ -9,11 +9,11 @@ import "fmt"
 
 class xclass {
 	# note that `os` is not imported here
-	$aaa = if os.is_debian() { "bbb" }  else { "ccc" }
+	$aaa = if os.is_family_debian() { "bbb" }  else { "ccc" }
 
 	print "${aaa}" {
 		msg => "hello",
 	}
 }
 -- OUTPUT --
-# err: errSetScope: func `os.is_debian` does not exist in this scope
+# err: errSetScope: func `os.is_family_debian` does not exist in this scope

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",