Introduce package "notices"

[mattpolzin]

I'm putting this implementation up for discussion. I don't know if the feature is desirable or if the name "notice" is the best choice.

The idea is that a package that was previously submitted to the pack collection might become defunct or have other important caveats worth telling users of the package about. For example, I want to move the FVect type from my fvect package into the containers package. I'm not deleting the FVect repository and there's no immediate need to remove it from the pack database -- removing it would potentially break downstream projects without warning. Instead, I would like to archive the GitHub repository and add a "notice" to the pack database entry that instructs consumers to find FVect in the containers package instead.

I especially don't know the most effective place in pack's various processes to surface this notice to the end-user of pack so I just picked a spot that gets hit when installing a package.

[stefan-hoeck]

This is a great idea! I have a couple such packages myself, but it never occured to me this could be a warning in pack itself.