Add a `--deps` flag to `bst checkout`

Background

Elements currently need to be careful about their runtime dependencies, since bst checkout will unconditionally stage these. This is annoying when creating an element that should be deployable, because it might stage, for example, its base platform.

Task description

We should add a --deps flag to bst checkout.

Acceptance Criteria

Elements no longer need to avoid runtime dependencies if they should be deployed.