Add `--deps build` option to `bst checkout`

As discussed in
https://mail.gnome.org/archives/buildstream-list/2018-September/msg00064.html,
add `--deps build` option to `bst checkout`. This will allow users to
checkout the all build dependencies of a given element using a single
command.

This change also fixes an existing issue where `--deps none` always
resulted in an empty output. (#670)

Fixes #670.