Fix nowarn in both old & new sbt plugin styles

Fixes #6430.

What is the problem?
As detailed in #6430, the @nowarn annotation was not suppressing
warnings, even after the first attempt to fix this in PR#6431.

This first PR fixed the problem for projects using
enablePlugins(SbtPlugin), but not for those using sbtPlugin := true.

Why is this a valuable problem to solve?
The annotation was not working as users would expect.

What is this solution?
I have moved the scalacOptions change from SbtPlugin.projectSettings
to the scalacOptions in the JvmPlugin settings.

Has this been tested?
Yes, a test has been added. Also, this branch was tested successfully
on the twinagle repo (https://github.com/soundcloud/twinagle/pull/224).

main/src/main/scala/sbt/Defaults.scala

@@ -959,6 +959,13 @@ object Defaults extends BuildCommon {
         Vector("-Ypickle-java", "-Ypickle-write", converter.toPath(earlyOutput.value).toString) ++ old
       else old
     },
+    scalacOptions := {
+      val old = scalacOptions.value
+      if (sbtPlugin.value && VersionNumber(scalaVersion.value)
+            .matchesSemVer(SemanticSelector("=2.12 >=2.12.13")))
+        old ++ Seq("-Wconf:cat=unused-nowarn:s")
+      else old
+    },
     persistJarClasspath :== true,
     classpathEntryDefinesClassVF := {
       (if (persistJarClasspath.value) classpathDefinesClassCache.value

main/src/main/scala/sbt/plugins/SbtPlugin.scala

@@ -8,24 +8,13 @@
 package sbt
 package plugins
 
-import Keys._
-import Def.Setting
-import sbt.SlashSyntax0._
-import sbt.librarymanagement.Configurations.Compile
-import sbt.librarymanagement.{ SemanticSelector, VersionNumber }
+import sbt.Def.Setting
+import sbt.Keys._
 
 object SbtPlugin extends AutoPlugin {
   override def requires = ScriptedPlugin
 
   override lazy val projectSettings: Seq[Setting[_]] = Seq(
-    sbtPlugin := true,
-    Compile / scalacOptions ++= {
-      // silence unused @nowarns in 2.12 because of https://github.com/sbt/sbt/issues/6398
-      // the option is only available since 2.12.13
-      if (VersionNumber(scalaVersion.value).matchesSemVer(SemanticSelector("=2.12 >=2.12.13")))
-        Some("-Wconf:cat=unused-nowarn:s")
-      else
-        None
-    }
+    sbtPlugin := true
   )
 }

sbt-app/src/sbt-test/project/sbt-plugin/changes/oldSbtPlugin.sbt

@@ -0,0 +1,6 @@
+lazy val root = project.in(file("."))
+  .settings(
+    scalaVersion := "2.12.13",
+    sbtPlugin := true,
+    scalacOptions ++= Seq("-Xfatal-warnings", "-Xlint")
+  )

sbt-app/src/sbt-test/project/sbt-plugin/test

@@ -1 +1,3 @@
 > compile
+$ copy-file changes/oldSbtPlugin.sbt build.sbt
+> compile