"bst track" of project.conf included junctions does not work

Summary

With include directive branch, I forgot to add a test for tracking junctions. And I broke the behavior before merge. It is impossible to track a junction if that junction was used in an include directive in project.conf.

Steps to reproduce

Have a junction used in an include directive in project.conf.

What is the current bug behavior?

The junction is tracked.

What is the expected correct behavior?

You get a complain that you should track the junction.

[--:--:--][][] START   Track
[--:--:--][][] START   Loading elements
[00:00:00][][] SUCCESS Loading elements
[--:--:--][][] START   Resolving elements
[00:00:00][][] SUCCESS Resolving elements
[00:00:00][][] FAILURE Track

Subproject has no ref for junction: junction.bst

    Try tracking the junction element with `bst track junction.bst`

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2

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