Resolve min-version in project.conf across internal junctions and suggest to update it

Currently, if project.conf has a `min-version: 2.3` (say) and some internal junctions of plugins:

```
name: foobar

min-version: 2.3

element-path: elements

junctions:
- plugins/buildstream-plugins.bst
```

If `plugins/buildstream-plugins.bst` is updated to a version that requires higher `min-version` than that the project itself specifies in `project.conf`, `bst` does not suggest or warn that `min-version: 2.3` needs to be updated, but it should.

If the junction plugins start asking for a higher `min-version` then that _effectively_ becomes the `min-version` of the project.conf, otherwise the project can be loaded.

Showing a warning or an error in this case would help people make a conscious decision of when/how to update internal junctions.

Start by tracing how project.conf reads min-version and how internal junctions are loaded and version requirements are checked. Confirm the behavior when a junction requires a higher version, then add coverage and ensure bst warns or errors clearly when the project's declared min-version must be updated.

Written by the indexing model from the issue text.