Refactor import paths and support python packages as sources

[fpacifici]

We decided to package jsonnet libraries as static resources in
jsonnet packages.

The recommended way in python >3.10 to load statics resources is via
importlib.resources.files(). This PR introduce an abstarction around
it to wire it up in the jsonnet module.

The issue with importlib.resources is that it does not necessarily
resturns Path objects. It returns Traversable objects, so we need
to bridge the gap. Thus the abstraction.

Also refactored the callback method into their own module that made
thinks easier and more testable.
Also added tests.