Have CI report if a circular dependency is introduced

[clalancette]

When we go to release a new distribution (like we are doing right now for Eloquent), we often find a couple of new circular dependencies have been introduced between repositories. Part of the reason for this is that there is no tool that tells us when this happens during the normal course of development.

What I'm thinking about is a tool that can tell us this during CI runs. The basic properties I'm thinking of are:

1. Probably a colcon plugin (since it has the topological information)
2. Run it at the end of CI, right after colcon test-result
3. Have some sort of whitelist of "allowed" circular dependencies, since we solve some of them via release-time ignore files. This particularly happens with some of the "test" packages that we don't release into the distribution.

Having this tool would mean that we can find out about the circular dependencies at the time of their introduction, not several months later when we go to release.