cc_toolchain: Fixes normalised include bug

This fixes the relative path prefix normalisation where the include path
was not properly normalised when including from a remote repository.

cc_toolchain/cc_toolchain_import.bzl

diff --git a/cc_toolchain/cc_toolchain_import.bzl b/cc_toolchain/cc_toolchain_import.bzl
index e7c6659..5cc65d9 100644
--- a/cc_toolchain/cc_toolchain_import.bzl
+++ b/cc_toolchain/cc_toolchain_import.bzl
@@ -57,7 +57,10 @@
     root_str = ""
     if ctx.label.workspace_root:
         root_str = ctx.label.workspace_root + "/"
-    return root_str + ctx.label.package + inc
+    package_str = ""
+    if ctx.label.package:
+        package_str = ctx.label.package + "/"
+    return root_str + package_str + inc
 
 def _cc_toolchain_import_impl(ctx):
     deps = ctx.attr.deps