REACH-70 Turn GetDefinitionLineIndexMap method into static helper method in CodeBlockUtils.

Also new test cases were written to test this new helper method in various combinations.
Thanks for asking, @pbidenko, that is a good point! As much as possible, we should return ```IEnumerable```. That not only hide the internal specifics away, but also the caller cannot modify the content of the list (if we return ```List```, then the caller can alter the list).