Gradually moving towards a unified CLI Core API module

[troydai]

It is not ideal to depend on private module as it is subject to change.

https://github.com/Microsoft/vscode-azurecli/blob/7d5817590648925f493e312ede67f0391db6f5bf/service/azservice/tooling1.py#L21

We're introducing a api module in the core so as to lock down the common APIs. The module will serve as the contact between command modules developers and the Core so that we know when to lock when a potential breaking change will be introduced.

Azure/azure-cli#6592.

When the next Core release is shipped, this extnesion can begin moving dependences on private module to this public module.