Clarify the structure of packagedcode.models.Package subclasses

[pombredanne]

This structure and how detection/recognition happens is not always obvious.
Also there are possible challenges when "rehydrating" a package plain data to a Package object.

We would need a doc and some refactoring as having several package manifests extensions for a single Package subclass makes things complicated.

Some redesign needed too in light of tracking package instances vs. manifests.

[pombredanne]

This has been completed. We now have a simpler structure:

• one class for each DatafileHandler, e.g. a manifest format parser and handler
• a single class for PackageData to keep track of parsed manifest data at the file level
• a single class for Package and Dependency to keep track of assembled manifest data at the top level
  Closing this now.