make helpers nonpublic

[jkowalleck]

some internal/helper classes are made public for no reason ... lets make them internal/nonpublic API.

• models.ComparableTuple
• model.sha1sum
• model.get_now_utc
• model.dependency.DependencyDependencies
• ...

iplementation:

• moved these classes to an own package/module, that starts with an underscore(_).
• have a pydoc in the package/module that describes the taht these are internal and what this means.
• import these private symbols by aliassing with underscore(from ._foo import bar as _bar)