fix#9189
Obviously this code is intended to throw an error if someone tries to reference a class before it's defined, like:
```js
class Foo {
someField = Foo;
}
```
But there's no problem with referencing the class in a type annotation before it's defined, and this is often necessary for tree structures:
```js
class Foo {
[someSymbol]: Foo;
}
```
* Make @babel/plugin-class-features a normal helper package
This effectively disallows using it directly.
* Rename helper
* Style
* Don't add prefix to plugin name
* Move private methods plugin
