Wrong type ouput for `getEffectiveModelType`

For the following TypeSpec, when we call `getEffectiveModelType` for type `Product`, we will get a type named `ProxyResource` with the same properties of the original type `Product`. I suppose it should return the original type of `Product`.
```
model Product is ProxyResource {
@doc("Name of product.")
@pattern("^[\\w][\\w\\s]{1,48}[\\w]$|^\\.default$|^\\.unassigned$")
@key("productName")
@path
@segment("products")
name: string;
}
```

This output may impact many things, include the output of `getLroMetadata` for Azure: https://github.com/Azure/typespec-azure/issues/311.