[sbt 2.0] Cross-build sbt-bloop with the 2.x milestones (Scala 3)

[kubukoz]

Context:

• sbt/sbt#7698

• https://github.com/sbt/sbt/wiki/sbt-2.x-plugin-migration

• Prerequisite: scalacenter/bloop-config#101

I did a first attempt to see how much is left: https://gist.github.com/kubukoz/af49151a8d27620f1dff556754297c39 , current issues look like:

• missing parens around lambda parameters with a type
• in syntax is still used instead of /
• some legit issues / renames like value Pure is not a member of sbt
• some file-related stuff, presumably because of sbt's new approach to remote caching
• missing givens, import sbt.given will likely be necessary
• indent warning, could be a false positive