Merge ppa-dev-tools:add_dependent_packages_function into ppa-dev-tools:main

This branch finally implements the core mechanics of this feature, the mapping of provided binary packages of some source packages to the reverse dependencies of others. The dependent_packages() API is introduced to provide a way to look up for a given source packages, the other packages that should be tested along with it.

I mulled over a few different ideas for the API name before setting on "dependent_packages()" but am still unsure that's the best name. Alternative ideas would be welcome, and I reserve the option to rename this later if inspiration strikes. :-)

This API will be used to generate all the various trigger URLs. I wanted to include a smoketest to showcase this, but this is already plenty to review.

This branch just introduces the core data structure and basic lookups. It doesn't handle recursive lookups and some corner cases like packages with no build dependencies, but those enhancements should be straightforward modifications, and will be tackled in a future MP.