Python Emitter should detect the missing dependencies for un-branded scenario

[lirenhe]

Clear and concise description of the problem

In order to let vscode typespec extension report the missing dependencies to run codegen, C# Emitter should have the ability to detect the missing dependencies in the target machine for un-branded scenario and return the correct message.

Please check for more details: https://github.com/microsoft/typespec/issues/5166

Checklist

• [x] Follow our Code of Conduct
• [x] Read the docs.
• [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.