[TCGC] while alter property type to ManagedServiceIdentity, TCGC returns unknown

[live1206]

Playgound
For csharp, we use alternateType to update a property type to Azure.ResourceManager.Foundations.ManagedServiceIdentity, but tcgc returns the property type as unknown.
While alternate the type to Azure.Core types, such as uuid, it works.

[tadelesh]

This is a regression introduced in this PR. This judgement is not enough. We need to add a build-in external type template in the TCGC and if the alternate is a model from the template, then it is an external model, else, it is just a normal alternate type.