Better coordination between sbt-scalafmt & sbt-scalafix when scalaf..OnCompile := true

[bjaglin]

Expanding on @olafurpg 's idea:

• sbt-scalafix would always execute callbacks registered in the task key scalafixHooks
• when both scalafmtOnCompile := true &&scalafixOnCompile := true, sbt-scalafmt wouldn't run scalafmt before compile, but instead in scalafixHooks (which would be "fed" via a dynamic task)

[bjaglin]

/cc @poslegm - I can take the lead on that in the coming days

[bjaglin]

Ideally we should first cut a release with the hooks to be able to write scripted tests with sbt-scalafix in sbt-scalafmt.

[bjaglin]

In scalacenter/sbt-scalafix#147 (comment), @olafurpg wrote:

I think it's also worth exploring another approach where we have a scalafmt rule that formats the code and we extend .scalafmt.conf to support running scalafix in several independent steps. This shouldn't be too hard to support when combined with #1204

If I get it right, your idea is integrate scalafmt into scalafix (and potentially scalafix into scalafmt, although that sounds redundant?), in the core rather than in the sbt plugins.

I'll start exploring the idea of exposing a special built-in Scalafmt scalafix rule backed by scalafmt-dynamic, that would after all patches are applied.

• sbt-scalafix should include .scalafix.conf in the cache checksum to avoid
• sbt-scalafix could detect the presence of sbt-scalafmt in the classpath via task introspection and suggest using that rule for better results (which was the starting point of this ticket)