Sort charms based on deploy-priority set in charm class. Refactor charm importing.

The main functional change here is to add the deploy-priority property and to deploy jujugui first.
The other changes are for clarity and to avoid reimplementing parts of the python standard library.
Details below:

- Import charm classes once at top of _process and sort by deploy-priority class variable.
- Use standard python importlib and pkgutil modules to import classes,
and use explicit __charm_class__ variable in charm modules to avoid
assuming that the first class in the module is the charm.
- Remove now-unused import helper functions in utils.