Consider making "built-in" complex types more strongly typed about scale-info

[Robbepop]

We could make the path representable as builtin which is only allowed from within the scale-info crate. So user defined types cannot have a builtin path. The set of types with a builtin path must be fixed and known upfront - e.g. Option<T>, Result<T, E>, Range, ... This way we avoid another huge set of ever growing enum variants for every builtin type but third party tools have a simple and efficient way of detecting an actually builtin type and differentiating one from a user defined type.
This kinda is possible nowadays with the prelude path if we were able to stop users from defining types with such a path.

Instead of making path more complex by introducing a builtin path we could also simply use a bool flag is_builtin that defaults to false and is not serialized therefore for all non-builtin types.

[ascjones]

At the moment all those types are in the "prelude" namespace (i.e. no namespace in the path), but there is nothing at the moment to stop a custom type being constructed with an empty namespace.

Anyway I have already released 1.0 now so added this to the 2.0 milestone. It's not a showstopper but we can certainly improve the experience here.