Fixes resource_strip_prefix on Windows

src/java/io/bazel/rulesscala/scalac/ScalacProcessor.java

@@ -291,11 +291,11 @@ private static void copyResources(
         dstr = resource.destination;
       }
 
-      if (dstr.charAt(0) == '/') {
+      if (dstr.charAt(0) == '/' || dstr.charAt(0) == '\\') {
         // we don't want to copy to an absolute destination
         dstr = dstr.substring(1);
       }
-      if (dstr.startsWith("../")) {
+      if (dstr.startsWith("../") || dstr.startsWith("..\\")) {
         // paths to external repositories, for some reason, start with a leading ../
         // we don't want to copy the resource out of our temporary directory, so
         // instead we replace ../ with external/

test/resource_strip_prefix/BUILD

@@ -0,0 +1,10 @@
+load("//scala:scala.bzl", "scala_library")
+
+scala_library(
+    name = "test-lib",
+    srcs = glob(
+        ["src/test/scala/**/*.scala"],
+    ),
+    resource_strip_prefix = "test/resource_strip_prefix/src/test/resources",
+    resources = ["src/test/resources/resource.txt"],
+)

test/resource_strip_prefix/src/test/scala/RootResources.scala

@@ -0,0 +1,3 @@
+object RootResources {
+
+}