package_publish: Default to component/section/priority in packages

For simple Debusine-managed archives, a good default is to just use the values of these fields from packages; we certainly don't want them to have to be set in suite_variables for every publication, not least because the desired values may differ between binary packages from the same source package.

We'll eventually need to implement proper override handling, but that can come later.

Part of #577 (closed).