Introduce the concept of a "successor package" to the purldb data model

It is not a super-common situation (fortunately) but sometimes a package gets moved and/or renamed to an entirely new namespace. Consider log4j

`pkg:maven/log4j/log4j@1.2.27` is followed by
`pkg:maven/org.apache.logging.log4j/log4j@2.0`

indicating that it became part of a "logging" project. This is fine, but it can make it challenging to find the "next" or "later" versions of a specific package.

Modeling details to be determined: possibly a new field on the basic package definition called `successor_package` (purl format of course) or possibly a new relation, since this is a relatively rare occurrence.