`--deps none` does not work for `bst checkout`
## Summary
[//]: # (Summarize the bug encountered concisely)
`bst checkout --deps none element.bst directory` currently produces empty output regardless of the element and the directory specified.
## Steps to reproduce
[//]: # (How one can reproduce the issue - this is very important)
```bash
$ cd tests/integration/project
$ cat > elements/hello.bst <<EOF
kind: import
build-depends:
- base.bst
sources:
- kind: local
path: files/workspace-mount-src/
EOF
$ bst build hello.bst
# Works as expected
$ bst checkout hello.bst deps && find deps
deps
deps/hello.c
# BUG - produces empty directory
$ bst checkout --deps none hello.bst deps-none && find deps-none
deps-none
```
## What is the current bug behavior?
[//]: # (What actually happens)
Empty directory is produced as output.
## What is the expected correct behavior?
[//]: # (What you should see instead)
With `--deps none` specified, just the specified element should be checked out.
## Relevant logs and/or screenshots
[//]: # (Paste any relevant logs - please use code blocks ``` to format console output, logs, and code as it's hard to read otherwise. You can also add a Snippet and link it here. Check the markdown giude on Gitlab for further tips)
## Possible fixes
[//]: # (If you can, link to the line of code that might be responsible for the problem)
## Other relevant information
[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)
* BuildStream version affected: /milestone %BuildStream_v1.x
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issue