Figure out how to keep the changelog up to date

I'm going to skip generating the changelog in tests, since it's annoying to have the tests fail if you forgot to include a commit type in the message. I should figure out a way to keep the changelog up to day, and make sure I get a sensible error if I try to package without a good changelog.

No files or tests are named. Start by tracing how changelog generation is used in tests and how packaging validates the changelog. Done means the changelog stays up to date and packaging reports a sensible error when its contents are insufficient.

Written by the indexing model from the issue text.