Improve indexing of Maven repos or repos subsets

As a follow up to https://github.com/nexB/scancode.io/issues/720 and to support https://github.com/nexB/scancode.io/issues/900 we need to support skinny indexing of the whole Maven Central, subsets of it and alternative Maven repos

Using a whole maven index and minecode, we can crawl the whole Maven central The outcome is that we can have "skinny" packages with only purl+sha1 for EVERYTNING in Maven central. We could also do this for other Nexus-based Maven repos. We can also handle non-nexus based Maven repos using maven-metadata.xml

We need to improve these capabilities such that:

1. Indexing a whole nexus-based Maven repo like Central is streamlined and works faster (breaking the visit/map thing to do it at once)
2. Ensure that we continuously keep up to date with the incremental updates of these nexus repo(s)
3. Allow configuration to support other nexus repos
4. Enable crawling only a subset of Maven Central using maven-metadata.xml e.g, focus the crawling on a subset of a whole Maven repo, such as org/apache/tomcat.
5. Enable crawling arbitrary Maven repos using maven-metadata.xml using configuration and not only Maven central.
6. Once we have indexed a skinny JAR, we would need a simple way to trigger full indexing when he skinny purl is matched as outlined in https://github.com/nexB/scancode.io/issues/900