refactor(mlablocate): use interface, remove unused fields, add docs (#383)

This is a very light refactoring of the mlablocate package where we do
the following things:

1. use interfaces rather depending on other pkgs where possible

2. only keep the fields we really need in the result struct

3. write more comprehensive docs (including todo-next comments)

While there, use `neubot/dash` rather than `ndt7` for the tests.

internal/engine/internal/mlablocate/mlablocate_test.go

@@ -17,7 +17,7 @@ func TestWithoutProxy(t *testing.T) {
 		log.Log,
 		"miniooni/0.1.0-dev",
 	)
-	result, err := client.Query(context.Background(), "ndt7")
+	result, err := client.Query(context.Background(), "neubot/dash")
 	if err != nil {
 		t.Fatal(err)
 	}