Create a focused visitor for maven

In the context of https://github.com/nexB/scancode.io/issues/900, it would be useful to have a focused visitor for maven that can visit and index the entire maven index (and other repos other than Maven Central). This visitor would be able to visit and index subsets of the repo, instead of collecting the entirety of the index from an arbitrary point.

As we visit, we can create packages with minimal package information (purl+sha1). This would help us identify maven packages by sha1, which we then can go scan and index fingerprints of the package.