8056252: Incremental build fails on Windows

Changed from '/' to File.separatorChar

Reviewed-by: jfranck
diff --git a/langtools/src/jdk.compiler/share/classes/com/sun/tools/sjavac/Source.java b/langtools/src/jdk.compiler/share/classes/com/sun/tools/sjavac/Source.java
index ad94ef4..2dea9b7 100644
--- a/langtools/src/jdk.compiler/share/classes/com/sun/tools/sjavac/Source.java
+++ b/langtools/src/jdk.compiler/share/classes/com/sun/tools/sjavac/Source.java
@@ -313,7 +313,7 @@
             int sp = fn.lastIndexOf(File.separatorChar);
             String pkg = "";
             if (sp != -1) {
-                pkg = fn.substring(0,sp).replace('/','.');
+                pkg = fn.substring(0,sp).replace(File.separatorChar,'.');
             }
             // Is this a module-info.java file?
             if (fn.endsWith("module-info.java")) {