Handle usage of dirPathTrailingSlash in DirectoryPath

`dirPathTrailingSlash` is used both in `DirectoryPath` and `ZipArchiveFileLookup`,
the former is platform-depndent, while the latter always uses `/` as separator.

Previously, the code works by accident: `new JFile(dir, path)` and `dir.resolve(path)`
are able to handle `path = "java/lang"` on windows.

Author: liufengyun

compiler/src/dotty/tools/dotc/classpath/ClassPath.scala

@@ -23,8 +23,15 @@ trait SourceFileEntry extends ClassRepresentation {
 }
 
 case class PackageName(dottedString: String) {
+  val dirPathTrailingSlashJar: String = FileUtils.dirPathInJar(dottedString) + "/"
+
+  val dirPathTrailingSlash: String =
+    if (java.io.File.separatorChar == '/')
+      dirPathTrailingSlashJar
+    else
+      FileUtils.dirPath(dottedString) + java.io.File.separator
+
   def isRoot: Boolean = dottedString.isEmpty
-  val dirPathTrailingSlash: String = FileUtils.dirPathInArchive(dottedString) + "/"
 
   def entryName(entry: String): String = {
     if (isRoot) entry else {

compiler/src/dotty/tools/dotc/classpath/FileUtils.scala

@@ -44,7 +44,7 @@ object FileUtils {
 
   def dirPath(forPackage: String): String = forPackage.replace('.', JFile.separatorChar)
 
-  def dirPathInArchive(forPackage: String): String = forPackage.replace('.', '/')
+  def dirPathInJar(forPackage: String): String = forPackage.replace('.', '/')
 
   def endsClass(fileName: String): Boolean =
     fileName.length > 6 && fileName.substring(fileName.length - 6) == ".class"

compiler/src/dotty/tools/dotc/classpath/ZipArchiveFileLookup.scala

@@ -62,7 +62,7 @@ trait ZipArchiveFileLookup[FileEntryType <: ClassRepresentation] extends Efficie
     }
 
   private def findDirEntry(pkg: PackageName): Option[archive.DirEntry] =
-    archive.allDirs.get(pkg.dirPathTrailingSlash)
+    archive.allDirs.get(pkg.dirPathTrailingSlashJar)
 
   protected def createFileEntry(file: FileZipArchive#Entry): FileEntryType
   protected def isRequiredFileType(file: AbstractFile): Boolean