This removes the dependency on online repositories (on GitHub)
and speeds up the test by stripping the number of versions and files to a minimum.
Prepare the ground for RULEAPI-753.
The main changes are:
* Split RuleCreator: move some of its content to RspecRepo and to
RuleEditor in new modules.
* Refactor tests accordingly.
Other less important changes:
* Sort and remove unnecessary imports
* Remove unimplemented functions and unnecessary classes
* Make some functions private
* Move pushd from utils to tests where it is only used
* Reduce code duplication here and there
* Remove unnecessary Mock in some tests
* Improve coverage for add_language_to_rule
