sbt: scalafixConfig setting doesn't work

[steinybot]

Setting the scalafixConfig setting in sbt does not work and causes mysterious errors.

The error you get is:

sbt:bug-reports> scalafix --check
error: The Scala compiler option "-Ywarn-unused" is required to use RemoveUnused.
To fix this problem, update your build to use at least one Scala compiler
option that starts with "-Ywarn-unused"
[error] (Compile / scalafix) scalafix.sbt.ScalafixFailed: CommandLineError
[error] Total time: 2 s, completed 13/12/2019 10:24:27 PM

And yet:

sbt:bug-reports> show scalacOptions
[info] * -Yrangepos
[info] * -Ywarn-unused
[success] Total time: 0 s, completed 13/12/2019 10:28:27 PM

There is a reproduction here https://github.com/steinybot/bug-reports/tree/scalafix/scalafixConfig-sbt.

I did a little debugging and there are some strange goings. The args look like they aren't getting set correctly at all. Something else that is noticeable is that scalafix.sbt.ScalafixPlugin#scalafixInputTask will use the default args to load the rules and check if it isSemantic which will be false and it will run scalafix.sbt.ScalafixPlugin#scalafixSyntactic. It doesn't even appear to use scalacOptions unless it runs scalafix.sbt.ScalafixPlugin#scalafixSemantic.

This is possibly related to #949.