cli: Interactively warn if the user is trying to close the workspace they're...

cli: Interactively warn if the user is trying to close the workspace they're using to load the project

This involves changes in:
* _stream.py:
  * Add the helper Stream.workspace_is_required()
* cli.py:
  * If buildstream is invoked interactively, prompt the user to confirm
    that they want to close the workspace they're using to load this
    project.