Cannot include yaml fragments in junction elements

[Cynical-Optimist]

See original issue on GitLab
In GitLab by [Gitlab user @tristanvb] on Jul 21, 2020, 11:25

Currently we have an undocumented limitation, which is that we fail to expand ([[Gitlab user @]](https://gitlab.com/)) files when these are declared inside a junction element.

This is not mentioned in the include documentation or in the junction documentation.

While it is possible to end up in unresolvable situations by allowing includes in junctions, it is perfectly possible to allow, we just need to have the correct early stage LoadErrors in place if ever an include leads to a circular reference into the same project that the given junction is loading. This error is already probably accounted for by LoadErrorReason.CIRCULAR_REFERENCE which is checked for and raised in the Loader.get_loader() API.

In the worst case, we need to document this as unsupported, but I think we should support this.