Support "related object" association

Occasionally, one has a situation such as:

* `foo`, a deprecated callable, which raises `_SomeKindOfException`

where `_SomeKindOfException` is raised only by `foo`, and perhaps isn't itself public.

Once `foo` is removed, it is potentially easy to forget to remove `_SomeKindOfException`.

It may be nice to provide a way to ensure `_SomeKindOfException` is marked as related to `foo` inside of `foo`'s deprecation. This may be so that e.g. #1 or #14 are able to raise errors if `foo` gets removed but `_SomeKindOfException` remains.

Start by reviewing the deprecation mechanism used by foo and the requirements discussed in issues #1 and #14. Define how a deprecated callable records its related exception and what checks should report when the callable is removed while the related object remains.

Written by the indexing model from the issue text.