Refactor License indexing

We should simplify indexing and caching by using a list of license directories be they builtin, "additional" or plugin-contributed.

See https://github.com/nexB/scancode-toolkit/pull/2979#pullrequestreview-1150782261

> I think we should refactor this a bit more such that indexing licenses and rules just takes a list of directories. Everything else should IMHO just use the things already loaded. And we can simplify the detection of builtins based on a list of builtin licenses... This would help keep the code tidier IMHO.