Rework how parent/ancestor pom information is collected and used during on-demand maven mining

[JonoYang]

I was running a script to fetch license information for maven packages without a declared license expression. This script uses code from minecode/visitors/maven.py. However, my script appeared to not update the declared license expression for those packages. I debugged a bit and found that we are running into the situation where the parent poms for a maven package have a different namespace than the the package we're looking at, and (in certain cases) is preventing us from getting a license expression.

We should look into relaxing this constraint: https://github.com/nexB/purldb/blob/main/minecode/visitors/maven.py#L258

@pombredanne suggests that we should ignore certain parent poms, like https://github.com/sonatype/oss-parents . Another idea would be to save maven packages that are parent poms to the db and add those to the package sets for packages that get its license from the parent.