Auto add builder factories for nested built_collection classes

When using BuiltMap nesting, is it not automatically created?

```
abstract class Calendar implements Built {
...
@BuiltValueField(wireName: 'calendar')
BuiltMap>>? get calendar;
...
}
```

automatically generated below
```
..addBuilderFactory(
const FullType(BuiltMap, const [
const FullType(String),
const FullType(BuiltMap, const [
const FullType(String),
const FullType(
BuiltList, const [const FullType(DayDTO)])
])
]),
() => new MapBuilder>>())
```

however, the below needs to be added manually.
```
..addBuilderFactory(
const FullType(BuiltMap, const [
const FullType(String),
const FullType(
BuiltList, const [const FullType(DayDTO)])
]),
() => new MapBuilder>())
..addBuilderFactory(
const FullType(
BuiltList, const [const FullType(DayDTO)]),
() => new ListBuilder())
```

It won't work unless you add it manually in the code above. So you have to add it manually, but it works.

Is there a way to automatically generate it?