Add "namespace" flag in Python emitter, and refactor "package-name"

[lmazuel]

Clear and concise description of the problem

Some update about namespaces, after involving Jeffrey in the mix, the current strategy is that:

• If "clientNamespace" is defined, follow it
• Else if "namespace" is defined in tspconfig.yaml, follow it
• Else (no conf), assume the client namespace is the same as TypeSpec namespace

This doesn't require a behavior change between Azure or unbranded, it just happens that in the Azure spec repo, "namespace" or "clientNamespace" is required (at least one).

For Python in practical terms, it means adding "namespace", and update the "package-name" behavior to be infered if "namespace" is provided. For instance, if "namespace" is "azure.foo", we should auto-infer that "package-name" is "azure-foo" unless "package-name" has been specificed explicitly