Add a `--deps` flag to `bst checkout`
Description
I have modified bst checkout to allow for running without staging dependencies
This resolves #425 (closed)
Changes proposed in this merge request:
- Add --deps to buildstream/_frontend/cli.py
- Add param to buildstream/_stream.py
- Add related logic to buildstream/element.py
- Add tests for new functionality
This merge request, when approved, will close issue/bug: #425 (closed)