_frontend/cli: Unify choices for --deps option

Standardize choices for `--deps` option for all commands that use it.

`bst build --deps build` in particular is quite handy when one is
preparing to open a build shell. This originally came up in
https://gitlab.com/BuildStream/buildstream/-/issues/685#note_105460896.

This also helps with the overall CLI consistency as part of #1068.
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