Add support for 'include if exists' to the tools

Add the IncludeRule and IncludeRuleset classes to the tools, and use them to handle include if exists rules. This will un-block !489 (merged)

Technically IncludeRule can also handle include rules, but this needs bigger changes in the tools so that the content of the included files is still checked.

Known issue: the tools won't "see"/honor rules in include if exists files yet, and might therefore ask superfluous questions.