Print type keyword when the type of an object should be printed

I would prefer to fix this somewhere in the printing logic and not in
the construction logic of the tree but I didn't find a way to
differentiate the Ident trees correctly in the printer. For

  object X { def x = 0 }

the Ident trees of `X.type` and `X.x` seem to have the same shape at the
beginning.

Fixes #1002233

Author: kiritsuku

org.scala-refactoring.library/src/main/scala/scala/tools/refactoring/transformation/TreeFactory.scala

@@ -64,7 +64,9 @@ trait TreeFactory {
 
   def mkReturn(s: List[Symbol]): Tree = s match {
     case Nil => EmptyTree
-    case x :: Nil => Ident(x) setType x.tpe
+    case x :: Nil =>
+      val ident = if (x.isModuleClass) Ident(newTermName(s"${x.name}.type")) else Ident(x)
+      ident setType x.tpe
     case xs =>
       typer.typed(gen.mkTuple(xs map (s => Ident(s) setType s.tpe))) match {
         case t: Apply =>
@@ -114,7 +116,7 @@ trait TreeFactory {
     }
 
     if (mods != NoMods) valOrVarDef setSymbol NoSymbol.newValue(name, newFlags = mods.flags) else valOrVarDef
-  }  
+  }
 
   def mkParam(name: String, tpe: Type, defaultVal: Tree = EmptyTree): ValDef = {
     ValDef(Modifiers(Flags.PARAM), newTermName(name), TypeTree(tpe), defaultVal)

org.scala-refactoring.library/src/test/scala/scala/tools/refactoring/tests/sourcegen/ReusingPrinterTest.scala

@@ -223,6 +223,23 @@ class ReusingPrinterTest extends TestHelper with SilentTracing {
         d.copy(tpt = newTpt) replaces d
     }}}
 
+  @Test
+  def add_type_keyword_to_return_type_when_it_represents_an_object() = """
+    package add_type_keyword_to_return_type_when_it_represents_an_object
+    object X {
+      def o = X
+    }
+    """ becomes """
+    package add_type_keyword_to_return_type_when_it_represents_an_object
+    object X {
+      def o: X.type = X
+    }
+    """ after topdown { matchingChildren { transform {
+      case d @ DefDef(_, _, _, _, tpt: TypeTree, _) =>
+        val newTpt = tpt setOriginal mkReturn(List(tpt.tpe.typeSymbol))
+        d.copy(tpt = newTpt) replaces d
+    }}}
+
   @Test
   def add_override_flag() = """
     package add_override_flag