Make report work with `CrossScalaVersions` and also add a `Scala version` column

[mdedetrich]

Currently when sbt-license-report generates a report, it only uses the current Scala version where as ideally we should aggregate the dependencies from all Scala versions as its possible for projects to resolve dependencies differently based on the Scala version. Similarly a Scala version column should also be added as an option for those projects which do happen to resolve dependencies differently based on CrossScalaVersion.

Thankfully we don't need to explicitly handle the common usecase where dependencies are the same across different Scala versions since https://github.com/sbt/sbt-license-report/pull/57 added the functionality that filters out all duplicates.