Add an exception for box::use() to infix spaces linter

[klmr]

As discussed in #1087, this is a draft to extend the infix_spaces_linter with an exception for lhs/rhs expressions inside a box::use() call, which represent import declarations that should resemble file paths, and thus don’t call for spaces around /.

My XPath-fu is fairly rusty so I’d appreciate a review of the implementation. I’ve opted not to modify the rule’s current XPath, instead filtering the resulting lint list by excluding all occurrences of / inside box::use(). The XML structure of the parsed expressions makes this somewhat awkward, because rather than being nested (as I naïvely expected), the surrounding box::use() call expression is adjacent to the parent node of the / expression.

I’m not 100% sure whether my way of testing for a box::use() call is maximally specific or whether it could lead to false positives … however, I can’t see how this would happen.

[AshesITR]

Thanks for submitting this PR!

[klmr]

Thanks, I’ve made adjustments in response to your two comments. 👍

[AshesITR]

LGTM, thanks again!