Reorganize purldb repo

[JonoYang]

We have a lot of related code in different places. For example, the minecode/visitor and minecode/mapper modules have code that is logically together, but is currently separated. Reorganizing the repo helps with clarity and focuses the specific workflow we want.

We should organize code to reflect the multi-level data collection we want to do:

1. Collect name, version, checksum
2. Collect metadata from registry
3. Collect package and scan

See also:

• RFC: Adopt scancode.io "pipeline" approach for visit/map/collect  #521
• Reorganize visitor/mapper code #283

[JonoYang]

A start would be to merge the visitor and mapper files of each repo type and then place that in a new module called miners

[JonoYang]

The minecode visitors and mapper modules have been combined into the miners module.