Fix TASTy source position printer

Now it properly shows that the sources form the position section are
references in the name table. This includes the coloring of the indices
and referenced names.

```diff
 source paths:
-         0: t/Test.scala
+         0: 21 [t/Test.scala]
```

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/tasty/PositionUnpickler.scala

@@ -18,7 +18,7 @@ class PositionUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName) {
 
   private var myLineSizes: Array[Int] = uninitialized
   private var mySpans: util.HashMap[Addr, Span] = uninitialized
-  private var mySourcePaths: util.HashMap[Addr, String] = uninitialized
+  private var mySourceNameRefs: util.HashMap[Addr, NameRef] = uninitialized
   private var isDefined = false
 
   def ensureDefined(): Unit = {
@@ -31,15 +31,14 @@ class PositionUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName) {
         i += 1
 
       mySpans = util.HashMap[Addr, Span]()
-      mySourcePaths = util.HashMap[Addr, String]()
+      mySourceNameRefs = util.HashMap[Addr, NameRef]()
       var curIndex = 0
       var curStart = 0
       var curEnd = 0
       while (!isAtEnd) {
         val header = readInt()
         if (header == SOURCE) {
-          val path = nameAtRef(readNameRef()).toString
-          mySourcePaths(Addr(curIndex)) = path
+          mySourceNameRefs(Addr(curIndex)) = readNameRef()
         }
         else {
           val addrDelta = header >> 3
@@ -64,9 +63,9 @@ class PositionUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName) {
     mySpans
   }
 
-  private[tasty] def sourcePaths: util.ReadOnlyMap[Addr, String] = {
+  private[tasty] def sourceNameRefs: util.ReadOnlyMap[Addr, NameRef] = {
     ensureDefined()
-    mySourcePaths
+    mySourceNameRefs
   }
 
   private[tasty] def lineSizes: Array[Int] = {
@@ -75,5 +74,5 @@ class PositionUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName) {
   }
 
   def spanAt(addr: Addr): Span = spans.getOrElse(addr, NoSpan)
-  def sourcePathAt(addr: Addr): String = sourcePaths.getOrElse(addr, "")
+  def sourcePathAt(addr: Addr): String = sourceNameRefs.get(addr).fold("")(nameAtRef(_).toString)
 }

compiler/src/dotty/tools/dotc/core/tasty/TastyPrinter.scala

@@ -194,12 +194,12 @@ class TastyPrinter(bytes: Array[Byte]) {
         sb.append(s": ${offsetToInt(pos.start)} .. ${pos.end}\n")
       }
 
-      val sources = posUnpickler.sourcePaths
+      val sources = posUnpickler.sourceNameRefs
       sb.append(s"\n source paths:\n")
       val sortedPath = sources.toSeq.sortBy(_._1.index)
-      for ((addr, path) <- sortedPath) {
+      for ((addr, nameRef) <- sortedPath) {
         sb.append(treeStr("%10d: ".format(addr.index)))
-        sb.append(path)
+        sb.append(nameStr(s"${nameRef.index} [${tastyName(nameRef)}]"))
         sb.append("\n")
       }