Fix loop over array to use for-of instead of for-in

[ghost]

I can't find any tests that this changes, but listFiles returns an array and fs.addFile expects a path, not an array index. Why is this not getting caught by our tests?

Also, @weswigham: I've been thinking we could add a lint rule where the object of a for-in loop would have to be of the Map type. What do you think?

[sandersn]

👍

[rbuckton]

👍

[weswigham]

@Andy-MS Re: Lint rule: Seems a bit strong. I'd rather just forbid using a for-in loop where the thing being iterated over is of type string[] (thereby not triggering any syntax errors through misuse).

For the record, we use for-in loops 13 times in the checker, 5 times in the harness (4 after this change), and once in services. All 13 in the checker and once in the services layer are on maps (or things with indexers at least), but the same is not the case of our iteration in the harness.

TBH, we do use the construct infrequently enough that we could just forbid for-in completely and add exceptions around where keeping it is prudent for performance.