Stop automatic installation of semanticdb-scalac

[olafurpg]

Currently, sbt-scalafix automatically addCompilerPlugin(semanticdb-scalac) and scalacOptions += "-Yrangepo". This automatic setup does not work for builds that use libraryDependencies := or scalacOptions :=. I propose we stop with the automatic installation and recommend users to manually setup the compiler plugin

addCompilerPlugin("org.scalameta" % "semanticdb-scalac" % scalafixScalametaVersion.value  % cross CrossVersion.full)
scalacOptions += "-Yrangepos"

Alternatively, it's possible to configure this step with the scalafixEnable command for a single sbt session.

[MasseGuillaume]

@olafurpg why we don't reuse the logic of scalafixEnable? If semanticdb-scalac don't add it, etc.

[olafurpg]

We currently enable the settings from scalafixEnable in ScalafixPlugin. The problem is that many build.sbt override the settings defined by ScalafixPlugin, making the setup break. The scalafixEnable command bypasses this problem because you have to run it manually in the sbt shell after the build is loaded.