Artifact globbing is unintuitive across junctions

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1125)
In GitLab by [[Gitlab user @BenjaminSchubert]](https://gitlab.com/BenjaminSchubert) on Sep 7, 2019, 19:39

## Summary

[//]: # (Summarize the bug encountered concisely)

In a project, if I do `bst artifact show *`, it will show me all the elements in my project.

however, if I do: `bst artifact show my_junction.bst:*`, it errors out telling me nothing was found.

I think bst should work seemlessly across junctions.

## What is the current bug behavior?

[//]: # (What actually happens)

Bst doesn't glob across junctions

## What is the expected correct behavior?

[//]: # (What you should see instead)

Bst globs the same across junctions and on the top level project

Reproduce the two commands in the issue: `bst artifact show *` and `bst artifact show my_junction.bst:*`. Start by tracing the artifact glob handling used by the `bst artifact show` entry point, then compare top-level and junction behavior. Done means equivalent globs return the expected artifacts across both project levels.

Written by the indexing model from the issue text.