Handle cross-build projects

[stringbean]

At the moment only libraries for the current Scala version are considered when checking dependencies.
This leads to an issue where running dependencyLockCheck will fail if you switch current versions.

Having a lock file for each Scala version would probably be the easiest solution (eg build.sbt_2.11.lock or build_2.11.sbt.lock).

[dmivankov]

It is currently possible to create lock file per version with something like

dependencyLockFile := { baseDirectory.value / s"build.scala-${CrossVersion.partialVersion(scalaVersion.value) match { case Some((2, n)) => s"2.$n" }}.sbt.lock" },