Introduce a deprecation mechanism for supported packages

[parlough]

When we want to remove a supported package, for whatever reason that may be, right now we just have to remove it. That could disrupt developers, API doc authors, etc. who are relying on that package being present.

We should enable deprecating packages, perhaps showing a warning when its used and crossing it out on the "Pub package versions" list. Then in the next release, we can drop that package.