Add warning where multiple module remotes are used with the same org_path

[pinin4fjords]

Description of feature

The tools don't really allow installation of modules from multiple remotes with the same org_path. This occurs if you try to install modules from a vanilla fork of the modules repo (e.g. one you use for making PRs), while also using the main one.

Trying this does things like #2155 (comment) (see discussion there on why this is not supported), so we could do with an earlier warning highlighting the problem and the fact that we don't support that usage.