Searching for the workspace file is preferred to searching for the
project file for the moment, because it avoids the circumstance of
the workspaces being inside a directory where a higher-level directory
may have project.conf (but may not be a buildstream project).

This will benefit from being revisited when support for multiple
projects owning a workspace is supported - if we're inside a project and
running commands in a workspace that refers to multiple projects, we
want the one we're inside, not whichever added it first.

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()
* userconfig.yaml:
  * Add a default value for prompt.really-workspace-close-project-inaccessible
* _context.py:
  * Load the prompt 'really-workspace-close-project-inaccessible' from
    user config.
* 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.