WorkspaceLocal contains all the information needed to generate data
that's stored within the workspace. Currently this is just a list of
project directories.

i.e. with a given directory and filename, check parent directories until
either a directory with the filename is found, or you reach the root of
the filesystem.

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.

--force will skip confirmation prompts, and removing the workspace
you're currently using to load the project will fail without "--force".