When there are multiple sonarpedia.json files for the same language, the rules
picked up for the next sonarpedia.json file will override the ones picked up
previously. With this fix, the rules are not overwritten but all are collected,
so one repository can have multiple sonarpedia.json files for the same language.
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
