Add support for cross-rule references.

[pedramamini]

It's possible to reference rules from other ones:

https://yara.readthedocs.io/en/stable/writingrules.html#referencing-other-rules

Example:

This results in a failure on save due to failed rule validation:

Which of course is due to the fact that the referenced rule is not available. Thinking the best way to handle this is to test rules along with all other released-state rules.