fix session caching for the default scalafixInterface & parser

Regression introduced in 2ca68e4: to be memoized, settings must be
bound to a setting key and defined on a scope.

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -123,6 +123,20 @@ object ScalafixPlugin extends AutoPlugin {
       Invisible
     )
 
+  private val scalafixInterfaceProvider: SettingKey[() => ScalafixInterface] =
+    SettingKey(
+      "scalafixInterfaceProvider",
+      "Implementation detail - do not use",
+      Invisible
+    )
+
+  private val scalafixParser: SettingKey[Parser[ShellArgs]] =
+    SettingKey(
+      "defaultScalafixInterface",
+      "Implementation detail - do not use",
+      Invisible
+    )
+
   override lazy val projectConfigurations: Seq[Configuration] =
     Seq(ScalafixConfig)
 
@@ -147,7 +161,19 @@ object ScalafixPlugin extends AutoPlugin {
       )
     ),
     scalafixDependencies := Nil,
-    commands += ScalafixEnable.command
+    commands += ScalafixEnable.command,
+    scalafixInterfaceProvider := ScalafixInterface.fromToolClasspath(
+      scalafixScalaBinaryVersion.in(ThisBuild).value,
+      scalafixDependencies = scalafixDependencies.in(ThisBuild).value,
+      scalafixCustomResolvers = scalafixResolvers.in(ThisBuild).value
+    ),
+    scalafixParser := new ScalafixCompletions(
+      workingDirectory = baseDirectory.in(ThisBuild).value.toPath,
+      // Unfortunately, local rules will not show up as completions in the parser, as that parser can only
+      // depend on settings, while local rules classpath must be looked up via tasks
+      loadedRules = () => scalafixInterfaceProvider.value().availableRules(),
+      terminalWidth = Some(JLineAccess.terminalWidth)
+    ).parser
   )
 
   override def buildSettings: Seq[Def.Setting[_]] =
@@ -157,25 +183,6 @@ object ScalafixPlugin extends AutoPlugin {
 
   lazy val stdoutLogger = Compat.ConsoleLogger(System.out)
 
-  private val scalafixInterface: Def.Initialize[() => ScalafixInterface] =
-    Def.setting {
-      ScalafixInterface.fromToolClasspath(
-        scalafixScalaBinaryVersion.in(ThisBuild).value,
-        scalafixDependencies = scalafixDependencies.in(ThisBuild).value,
-        scalafixCustomResolvers = scalafixResolvers.in(ThisBuild).value
-      )
-    }
-
-  private val parser: Def.Initialize[Parser[ShellArgs]] = Def.setting {
-    new ScalafixCompletions(
-      workingDirectory = baseDirectory.in(ThisBuild).value.toPath,
-      // Unfortunately, local rules will not show up as completions in the parser, as that parser can only
-      // depend on settings, while local rules classpath must be looked up via tasks
-      loadedRules = () => scalafixInterface.value().availableRules(),
-      terminalWidth = Some(JLineAccess.terminalWidth)
-    ).parser
-  }
-
   private def scalafixArgsFromShell(
       shell: ShellArgs,
       scalafixInterface: () => ScalafixInterface,
@@ -222,7 +229,7 @@ object ScalafixPlugin extends AutoPlugin {
   private def scalafixAllInputTask(): Def.Initialize[InputTask[Unit]] =
     // workaround https://github.com/sbt/sbt/issues/3572 by invoking directly what Def.inputTaskDyn would via macro
     InputTask
-      .createDyn(InputTask.initParserAsInput(parser))(
+      .createDyn(InputTask.initParserAsInput(scalafixParser))(
         Def.task(shellArgs => scalafixAllTask(shellArgs, thisProject.value))
       )
 
@@ -246,7 +253,7 @@ object ScalafixPlugin extends AutoPlugin {
   ): Def.Initialize[InputTask[Unit]] =
     // workaround https://github.com/sbt/sbt/issues/3572 by invoking directly what Def.inputTaskDyn would via macro
     InputTask
-      .createDyn(InputTask.initParserAsInput(parser))(
+      .createDyn(InputTask.initParserAsInput(scalafixParser))(
         Def.task(shellArgs => scalafixTask(shellArgs, config))
       )
 
@@ -276,7 +283,7 @@ object ScalafixPlugin extends AutoPlugin {
         val scalafixConf = scalafixConfig.in(config).value.map(_.toPath)
         val (shell, mainInterface0) = scalafixArgsFromShell(
           shellArgs,
-          scalafixInterface.value,
+          scalafixInterfaceProvider.value,
           projectDepsExternal,
           scalafixDependencies.in(ThisBuild).value,
           scalafixResolvers.in(ThisBuild).value,
@@ -306,7 +313,10 @@ object ScalafixPlugin extends AutoPlugin {
     }
   private def scalafixHelp: Def.Initialize[Task[Unit]] =
     Def.task {
-      scalafixInterface.value().withArgs(Arg.ParsedArgs(List("--help"))).run()
+      scalafixInterfaceProvider
+        .value()
+        .withArgs(Arg.ParsedArgs(List("--help")))
+        .run()
       ()
     }