Is there an easy way to get the relative path to a file WRT the Gradle root project in a `Rule`'s

visitPackageDirective

function?

It does not. But right now there's a PR that implements exactly that: https://github.com/detekt/detekt/pull/3319 the initial idea was for reporting proposes but you could use it.

Hopefully it will arrive soon.

Thanks for pointing out that PR to me, I'll continue commenting over there!