boulder: Make non /usr/ build artefact warnings more visible and add more context

[ermo]

We recently ran into an issue where some compat files for bash completions were not being included in our shipped bash-completion package, due to a lack of a fully stateless solution on our end.

Given that @joebonrichie built the package and @ikeycode later updated it, it stands to reason that, if two of our most experienced packagers both missed the current warning about non /usr files:

... then we might need to reconsider our boulder UX for when less experienced packagers inevitably hit this as well?

Ikey suggested we look into miette style warnings.