diktat-gradle-plugin shouldn't be using `files` for patterns

Either specify `fileTree` with wildcards and use files resolved by gradle
or use special gradle class [PatternFilterable](https://docs.gradle.org/current/javadoc/org/gradle/api/tasks/util/PatternFilterable.html) for this
or specify globs as string and feed them to ktlint
or detect kotlin plugin and use sources directly