Fix indexing of class generic definitions

Currently generics are indexed as class definitions

```dart
class Foo<T> {}
//        ^ scip-dart pub ... lib/`foo.dart`/T#
```

Since `T` isn't actually a class, it shouldn't be indexed as such

scip-typescript indexes the above the following way:

```ts
class Foo<T> {}
//        ^ scip-typescript npm ... lib/`foo.dart`/Foo#[T]
```

we should mirror this in scip-dart