Inconsistencies with external-reference warning about regal HOT 2 CLOSED

Thanks @asleire!

Yeah, the implementation of this rule is rather simplistic.. but I'm still surrpised by a few of these. We have a few rules that can't currently catch all violations — typically because it's really expensive to do so in some cases — but we'll always have zero tolerance for false positives.

In this case it looks like we're already doing an expensive walk, so finding more external references currently not reported seems doable.

I'll take a look.

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PR up to fix every issue reported other than func_2. I looked into that one briefly but it looks like it would require more effort than it's probably worth at this point. Might open another issue later to deal with some remaining items, but this level of coverage feels quite alright to me.

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