Checkout gives an empty directory when using --deps none

Summary

When running bst checkout --deps none I get an empty directory.

Steps to reproduce

I got this with the Freedesktop SDK:

$ git clone https://gitlab.com/freedesktop-sdk/freedesktop-sdk
$ cd freedesktop-sdk
$ git checkout 18.08  # at this time, this is 718ea88089644a1ea5b488de0b90c2c565cb75f8
$ bst build desktop/libepoxy.bst
$ bst checkout --deps none desktop/libepoxy.bst libepoxy

What is the current bug behavior?

The new libepoxy/ directory is empty.

What is the expected correct behavior?

The new libepoxy/ directory contains a checkout of the files built/installed by desktop/libepoxy.bst, without any of the dependencies.

Other relevant information

If I run bst checkout desktop/libepoxy.bst libepoxy I get the expected tree with libepoxy and its dependencies.

It really is the --deps none which does not work.

• BuildStream version affected: 1.2.1

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