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

[bbhtt]

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.