Fixes clean not cleaning files added to cleanFiles

Fixes #6395

The current code assumes cleanFiles are only directories. This fixes that.

main/src/main/scala/sbt/internal/Clean.scala

@@ -93,8 +93,9 @@ private[sbt] object Clean {
         val targetDir = (scope / target).?.value.map(_.toPath)
 
         targetDir.filter(_ => full).foreach(deleteContents(_, excludeFilter, view, delete))
-        (scope / cleanFiles).?.value.getOrElse(Nil).foreach { f =>
-          deleteContents(f.toPath, excludeFilter, view, delete)
+        (scope / cleanFiles).?.value.getOrElse(Nil).foreach { x =>
+          if (x.isDirectory) deleteContents(x.toPath, excludeFilter, view, delete)
+          else delete(x.toPath)
         }
 
         // This is the special portion of the task where we clear out the relevant streams

sbt/src/sbt-test/actions/clean-files-directory/build.sbt

@@ -1 +1,2 @@
-cleanFiles += baseDirectory.value / "foo"
+cleanFiles += baseDirectory.value / "foo"
+cleanFiles += baseDirectory.value / "a.txt"

sbt/src/sbt-test/actions/clean-files-directory/test

@@ -1,3 +1,4 @@
 > clean
 
-$ absent foo/bar/baz.txt
+$ absent foo/bar/baz.txt
+$ absent a.txt