Enhance Python package file collection

Currently, scancode creates package objects from package manifest files. It would be useful to look at other ancillary package files for more information to enhance the data used to create the package object. For example, when reporting a Python package from a `METADATA` file, we can also find the package files for that package from looking at the `RECORD` file that is in the same directory as `METADATA`. This idea can be generalized for other packages, where missing package data can be filled in from multiple sources that are alongside the main package manifest file.