RFC: Introduce idea of embedded package

Many times, we encounter packages within other packages. For example, there are maven JARs that contain JARs, as is the case with Spring Boot or other uber JARs. We currently do not have the concept of relating packages like this, where a group of packages are contained within another package. This relation would help us in the cases where we are matching an uber JAR package and we can have a list of all the packages contained within that package and return with the match not only the uber JAR package data, but also the package data for all the other embedded packages.

This will probably be a new foreign key on Packages with the name `embedded_packages`.