Handle maven monorepos in a better way

Right now in purlDB a single source URL points to a single maven package, but in case of maven we have a single monorepo source which contains many packages. One way to solve this problem could be to store subpaths inside source URL and the way to get source URL with the help of a maven and source purl could be:
- download the source code and get all *.pom files
- parse all the pom file and wherever we find the namespace as parent and name as artifact ID use that subdirectory as subpath