Introduce notion of Package set

We have the case where we have different instances of the same package. For example, for a maven Package, we can have the source, test, or doc JAR for that package. These JARs are part of the same package but would be considered different packages in the PackageDB, as they have different download URLs. These JARS would have similar purl values (same type, namespace, name, and version) but different subpath and qualifiers. We want to be able to relate this group of Packages together such that we can combine all the findings from the different varieties of a Package and return the data to the user.

After some discussion with @pombredanne, we will add two new fields to the Package model:
- `package_set` (working name)
- This field contains a uuid or unique identifier for a group of packages. This ID would be unique to a particular purl (type, namespace, name, version) combination. The ID is generated when a Package with a purl that is not yet in the PackageDB is created. When another package with the same type, namespace, name, and version is created, the ID would also be used here.
- `package_content`
- This field contains a tag that describes what is in this package, either source, binary, doc, tests, etc.

There would be an API endpoint (or equivalent), where we query for a package, then we would combine the metadata based on some set precedence (curated package data, source data data, then binary, etc) and return the combined package data to the user.

@pombredanne @DennisClark

I'd like to hear your thoughts on grouping like packages via ID