This ports sbt-cross-building's cross (`^`) and switch (`^^`) commands.
Instead of making it a plugin, the default settings are now changed
to use `sbtVersion in pluginCrossBuild` for the sbt dependency.
This was already supported in the internal Scripted used by sbt but not
in the ScriptedPlugin. This is fixed by just copy-pasting the modified
parser. We will have to wait for sbt itself to be built using an sbt
with the upgraded ScriptedPlugin to be able to avoid the code duplication.
All of `sbtVersion`, `defScalaVersion` and `buildScalaVersions` were
not used anymore. According to @harrah they are coming from really
old days of sbt and are not needed because of changes to how sbt
interacts with different Scala versions.