Use relative paths in position pickler

Improve state of #7474

Author: nicolasstucki

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

@@ -45,7 +45,9 @@ class PositionPickler(pickler: TastyPickler, addrOfTree: untpd.Tree => Addr) {
 
     def pickleSource(source: SourceFile): Unit = {
       buf.writeInt(SOURCE)
-      buf.writeInt(pickler.nameBuffer.nameIndex(source.pathName).index)
+      val jpath = source.file.jpath
+      // assert(!jpath.isAbsolute, jpath)
+      buf.writeInt(pickler.nameBuffer.nameIndex(jpath.toString.toTermName).index)
     }
 
     /** True if x's position shouldn't be reconstructed automatically from its initial span

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -78,7 +78,7 @@ class Pickler extends Phase {
         pickled.iterator.grouped(10).toList.zipWithIndex.map {
           case (row, i) => s"${i}0: ${row.mkString(" ")}"
         }
-        
+
       // println(i"rawBytes = \n$rawBytes%\n%") // DEBUG
       if (pickling ne noPrinter) {
         println(i"**** pickled info of $cls")

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -59,8 +59,6 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
   override def path: String = file.path
   override def jfile: Optional[JFile] = Optional.ofNullable(file.file)
 
-  def pathName: PathName = file.absolutePath.toTermName
-
   override def equals(that: Any): Boolean =
     (this `eq` that.asInstanceOf[AnyRef]) || {
       that match {