refactor(inputloader): better docs and naming (#265)

* refactor(inputloader): better docs and naming

Work done as part of https://github.com/ooni/probe/issues/1299.

* fix: correct a typo

internal/engine/experiment_integration_test.go

@@ -142,7 +142,7 @@ func TestNeedsInput(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if builder.InputPolicy() != InputOrQueryTestLists {
+	if builder.InputPolicy() != InputOrQueryBackend {
 		t.Fatal("web_connectivity certainly needs input")
 	}
 }