Feature request: generate in sub-namespaces for clients and models

[xiangyan99]

In Search service, we have two different namespaces: azure.search.documents & azure.search.documents.indexes.

Here are what we want to achieve:

1. We want to create two clients. One under azure.search.documents and the other under azure.search.documents.indexes.
2. We want to split the models into two namespaces. (What it matters here are the docs.)

Thanks.

[pshao25]

@xiangyan99 next time you could directly open up issue in autorest.python to get timely support.

[iscai-msft]

@xiangyan99 this is something we're actively looking to do across all sdks: by default namespaces correspond to namespaces in our generated packages