`--deps none` does not work for `bst checkout`

Summary

bst checkout --deps none element.bst directory currently produces empty output regardless of the element and the directory specified.

Steps to reproduce

$ 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?

Empty directory is produced as output.

What is the expected correct behavior?

With --deps none specified, just the specified element should be checked out.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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