Unify global caches (defaults to ~/.coursier/cache/v1)

cache/src/main/scala/coursier/Cache.scala

@@ -473,6 +473,13 @@ object Cache {
       "[artifact](-[classifier]).[ext]"
   )
 
+  lazy val defaultBase = new File(
+    sys.env.getOrElse(
+      "COURSIER_CACHE",
+      sys.props("user.home") + "/.coursier/cache/v1"
+    )
+  )
+
   val defaultConcurrentDownloadCount = 6
 
   lazy val defaultPool =

cli/src/main/scala-2.11/coursier/cli/Coursier.scala

@@ -45,18 +45,10 @@ case class CommonOptions(
   val verbose0 = verbose.length - (if (quiet) 1 else 0)
 }
 
-object CacheOptions {
-  def default =
-    sys.env.getOrElse(
-      "COURSIER_CACHE",
-      sys.props("user.home") + "/.coursier/cache"
-    )
-}
-
 case class CacheOptions(
-  @HelpMessage("Cache directory (defaults to environment variable COURSIER_CACHE or ~/.coursier/cache)")
+  @HelpMessage("Cache directory (defaults to environment variable COURSIER_CACHE or ~/.coursier/cache/v1)")
   @ExtraName("C")
-    cache: String = CacheOptions.default
+    cache: String = Cache.defaultBase.toString
 )
 
 sealed trait CoursierCommand extends Command

plugin/src/main/scala-2.10/coursier/CoursierPlugin.scala

@@ -39,7 +39,7 @@ object CoursierPlugin extends AutoPlugin {
     coursierVerbosity := 1,
     coursierResolvers <<= Tasks.coursierResolversTask,
     coursierSbtResolvers <<= externalResolvers in updateSbtClassifiers,
-    coursierCache := new File(sys.props("user.home") + "/.coursier/sbt"),
+    coursierCache := Cache.defaultBase,
     update <<= Tasks.updateTask(withClassifiers = false),
     updateClassifiers <<= Tasks.updateTask(withClassifiers = true),
     updateSbtClassifiers in Defaults.TaskGlobal <<= Tasks.updateTask(withClassifiers = true, sbtClassifiers = true),