[2.x] Emit warning for unused

[Friendseeker]

Let CI emit warning for unused import, unused variable etc.

[Friendseeker]

Actually this is a good change to have in 1.10.x branch. Maybe do it in 1.10.x first and then forward merge?

In 1.10.x we have -Xfatal-warnings on for Scala 2.12 build so may need to do some grunt work.

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Now I see why "-Ywarn-unused" was not enabled first... It sometimes have false positive

[error] D:\Repos\zinc\internal\zinc-classpath\src\main\scala\sbt\internal\inc\ReflectUtilities.scala:52:11: parameter value field in anonymous function is never used
[error]           field <- correspondingFields.get(method.getName)

I guess I shall take a break from linting for now. Having two branches & cross building against multiple Scala versions complicate things, and unused is far less important than deprecation warnings.

If anyone else is interested, feel free to takeover the PR.